Timing of cortical excitability changes during the reaction time of movements superimposed on tonic motor activity

Paired corticomotoneuronal stimulation of the preactivated ankle dorsiflexor: an open-label study of magnetic and electrical painless protocols

Paired corticomotoneuronal stimulation (or electrical PCMS: ePCMS) is the repetitive pairing of an electrical stimulus to a nerve with a transcranial magnetic stimulation of the primary motor cortex (TMS-of-M1) to noninvasively influence spinal plasticity. We compared ePCMS with the new painless PCMS protocol pairing a magnetic stimulus to the nerve with TMS-of-M1 (mPCMS) in the preactivated tibial anterior muscle (TA). Sixteen healthy adults participated in two sessions (mPCMS, ePCMS), each with 180 pairs of [low-intensity TMS-of-M1 + nerve stimulation] at 0.2 Hz. TA motor-evoked potentials (MEP) to single-pulse TMS at pre-PCMS, immediately and 30 min after PCMS, were cluster-analyzed to discriminate responders and non-responders. Paired-pulse TMS-of-M1 and F-waves were also tested and BDNF polymorphism influence was explored. Both PCMS protocols significantly increased MEP amplitudes (n = 9 responders each), but the time-course differed with mPCMS inducing larger MEP increase over time. The number of BDNF-methionine carriers tended to be larger than Val66Val in mPCMS and the reverse in ePCMS, thus warranting further investigations. The MEP changes of the preactivated TA likely occurred at the pre-motoneuronal level and larger mPCMS after-effects over time may be related to the afferents recruited. mPCMS seems relevant to be tested in future studies as a painless noninvasive approach to induce sustained pre-motoneuronal plasticity in spinal cord injury.

Spinal Cord Injury and Loss of Cortical Inhibition

After spinal cord injury (SCI), the destruction of spinal parenchyma causes permanent deficits in motor functions, which correlates with the severity and location of the lesion. Despite being disconnected from their targets, most cortical motor neurons survive the acute phase of SCI, and these neurons can therefore be a resource for functional recovery, provided that they are properly reconnected and retuned to a physiological state. However, inappropriate re-integration of cortical neurons or aberrant activity of corticospinal networks may worsen the long-term outcomes of SCI. In this review, we revisit recent studies addressing the relation between cortical disinhibition and functional recovery after SCI. Evidence suggests that cortical disinhibition can be either beneficial or detrimental in a context-dependent manner. A careful examination of clinical data helps to resolve apparent paradoxes and explain the heterogeneity of treatment outcomes. Additionally, evidence gained from SCI animal models indicates probable mechanisms mediating cortical disinhibition. Understanding the mechanisms and dynamics of cortical disinhibition is a prerequisite to improve current interventions through targeted pharmacological and/or rehabilitative interventions following SCI.

The Inhibition Effect of Affordances in Action Picture Naming: An ERP Study

How quickly are different kinds of conceptual knowledge activated in action picture naming? Using a masked priming paradigm, we manipulated the prime category type (artificial vs. natural), prime action type (precision, power, vs. neutral grip), and target action type (precision vs. power grip) in action picture naming, while electrophysiological signals were measured concurrently. Naming latencies showed an inhibition effect in the congruent action type condition compared with the neutral condition. ERP results showed that artificial and natural category primes induced smaller waveforms in precision or power action primes than neutral primes in the time window of 100-200 msec. Time-frequency results consistently presented a power desynchronization of the mu rhythm in the time window of 0-210 msec with precision action type artificial objects compared with neutral primes, which localized at the supplementary motor, precentral and postcentral areas in the left hemisphere. These findings suggest an inhibitory effect of affordances arising at conceptual preparation in action picture naming and provide evidence for embodied cognition.

Subcortical Contribution of Corticospinal Transmission during Visually Guided Switching Movements of the Arm

In animal experiments, the indirect corticospinal tract (CST) system via cervical interneurons has been shown to mediate motor commands for online adjustment of visuomotor behaviors, such as target-reaching. However, it is still unclear whether the similar CST system functions to perform similar motor behaviors in humans. To clarify this, we investigated changes in motor-evoked potentials (MEPs) in the elbow muscles following transcranial magnetic stimulation, transcranial electrical stimulation, or cervicomedullary stimulation while participants executed target-reaching and switching movements. We found that the MEP, whether elicited cortically or subcortically, was modulated depending on the direction of the switching movements. MEP facilitation began around the onset of the switching activities in an agonist muscle. Furthermore, ulnar nerve-induced MEP facilitation, which could be mediated by presumed cervical interneuronal systems, also increased at the onset of MEP facilitation. In a patient with cortical hemianopsia who showed switching movements in the scotoma, the MEPs were facilitated just before the switching activities. Our findings suggested that CST excitation was flexibly tuned with the switching movement initiation, which could partly take place in the subcortical networks, including the presumed cervical interneuronal systems.

Changes in corticospinal excitability during bilateral and unilateral lower-limb force control tasks

Ankle dorsiflexion force control is essential for performing daily living activities. However, the involvement of the corticospinal pathway during different ankle dorsiflexion tasks is not well understood. The objective of this study was to compare the corticospinal excitability during: (1) unilateral and bilateral; and (2) ballistic and tonic ankle dorsiflexion force control. Fifteen healthy young adults (age: 25.2 ± 2.8 years) participated in this study. Participants performed unilateral and bilateral isometric ankle dorsiflexion force-control tasks, which required matching a visual target (10% of maximal effort) as quickly and precisely as possible during ballistic and tonic contractions. Transcranial magnetic stimulation (TMS) was applied over the primary motor cortex to elicit motor-evoked potentials (MEPs) from the right tibialis anterior during: (i) pre-contraction phase; (ii) ascending contraction phase; (iii) plateau phase (tonic tasks only); and (iv) resting phase (control). Peak-to-peak MEP amplitude was computed to compare the corticospinal excitability during each experimental condition. MEP amplitudes significantly increased during unilateral contraction compared to bilateral contraction in the pre-contraction phase. There were no significant differences in the MEP amplitudes between the ballistic tasks and tonic tasks in any parts of the contraction phase. Although different strategies are required during ballistic and tonic contractions, the extent of corticospinal involvement appears to be similar. This could be because both tasks enhance the preparation for precise force control. Furthermore, our results suggest that unilateral muscle contractions may largely facilitate the central nervous system during movement preparation for unilateral force control compared to bilateral muscle contractions.

Premovement Suppression of Corticospinal Excitability may be a Necessary Part of Movement Preparation

In reaction time (RT) tasks corticospinal excitability (CSE) rises just prior to movement. This is preceded by a paradoxical reduction in CSE, when the time of the imperative (“GO”) stimulus is relatively predictable. Because RT tasks emphasise speed of response, it is impossible to distinguish whether reduced CSE reflects a mechanism for withholding prepared actions, or whether it is an inherent part of movement preparation. To address this question, we used transcranial magnetic stimulation (TMS) to estimate CSE changes preceding 1) RT movements; 2) movements synchronized with a predictable signal (predictive timing or PT movements); and 3) self-paced movements. Results show that CSE decreases with a similar temporal profile in all three cases, suggesting that it reflects a previously unrecognised state in the transition between rest and movement. Although TMS revealed reduced CSE in all movements, the TMS pulse itself had different effects on movement times. TMS given ~200 ms before the times to move speeded the onset of RT and self-paced movements, suggesting that their initiation depends on a form of trigger that can be conditioned by external events. On the contrary, PT movements did not show this effect, suggesting the use of a different triggering strategy prioritizing internal events.

A Central Component of the N1 Event-Related Brain Potential Could Index the Early and Automatic Inhibition of the Actions Systematically Activated by Objects

Stimuli of the environment, like objects, systematically activate the actions they are associated to. These activations occur extremely fast. Nevertheless, behavioral data reveal that, in most cases, these activations are then automatically inhibited, around 100 ms after the occurrence of the stimulus. We thus tested whether this early inhibition could be indexed by a central component of the N1 event-related brain potential (ERP). To achieve that goal, we looked at whether this ERP component is larger in tasks that could increase the inhibition and in trials where reaction times (RTs) happen to be long. The illumination of a real space bar of a keyboard out of the dark was used as a stimulus. To maximize the modulation of the inhibition, the task participants had to perform was manipulated across blocks. A look-only task and a count task were used to increase inhibition and an immediate press task was used to decrease it. ERPs of the two block-conditions where presses had to be prevented and where the largest central N1s were predicted were compared to those elicited in the press task, differentiating the ERPs to the third of the trials where presses were the slowest from the ERPs to the third of the trials with the fastest presses. Despite larger negativities due to lateralized readiness potentials (LRPs) and despite greater attention likely in immediate press-trials, central N1s were found to be minimal for the fastest presses, intermediate for the slowest ones and maximal for the two no-press conditions. These results thus provide a strong support for the idea that the central N1 indexes an early and short lasting automatic inhibition of the actions systematically activated by objects. They also confirm that the strength of this automatic inhibition spontaneously fluctuates across trials and tasks. On the other hand, just before N1s, parietal P1s were found larger for fastest presses. They might thus index the initial activation of these actions. Finally, consistent with the idea that N300s index late inhibition processes, that occur preferentially when the task requires them, these ERPs were quasi absent for fast presses trials and much larger in the three other conditions.

Cortical contributions to anticipatory postural adjustments in the trunk

KEY POINTS

Increases in activity of trunk muscles that occur prior to, or concurrent with, a voluntary limb movement are termed anticipatory postural adjustments (APAs). APAs are important for maintaining postural stability in response to perturbations but the neural mechanisms underlying APAs remain unclear. Our results showed that corticospinal excitability of erector spinae (ES) muscle increased at 40 ms prior to rapid shoulder flexion, with a reduction in intracortical inhibition and no change in spinal excitability. Changes in corticospinal excitability were observed in ES, with similar excitability profiles between standing and lying positions, but were not observed in rectus abdominis. We suggest that the neural control of postural adjustments involves changes at a cortical level, which in part are due to reduced inhibition.

ABSTRACT

Voluntary limb movements are associated with increases in trunk muscle activity, some of which occur within a time window considered too fast to be induced by sensory feedback; these increases are termed anticipatory postural adjustments (APAs). Although it is known that the function of APAs is to maintain postural stability in response to perturbations, excitability of the corticospinal projections to the trunk muscles during the APAs remains unclear. Thirty-four healthy subjects performed rapid shoulder flexion in response to a visual cue in standing and lying positions. Transcranial magnetic stimulation (TMS) was delivered over the trunk motor cortex to examine motor evoked potentials (MEPs) in erector spinae (ES) and in rectus abdominis (RA) muscles at several time points prior to the rise in electromyographic activity (EMG) of anterior deltoid (AD) muscle. TMS was also used to assess short-interval intracortical inhibition (SICI) and cervicomedullary MEPs (CMEPs) in ES in the standing position. MEPs in ES were larger at time points closer to the rise in AD EMG in both standing and lying positions, whereas MEPs in RA did not differ over the time course examined. Notably, SICI was reduced at time points closer to the rise in AD EMG, with no change in CMEPs. Our results demonstrate that increasing excitability of corticospinal projections to the trunk muscles prior to a voluntary limb movement is likely to be cortical in origin and is muscle specific.

Selective Suppression of Local Interneuron Circuits in Human Motor Cortex Contributes to Movement Preparation

Changes in neural activity occur in the motor cortex before movement, but the nature and purpose of this preparatory activity is unclear. To investigate this in the human (male and female) brain noninvasively, we used transcranial magnetic stimulation (TMS) to probe the excitability of distinct sets of excitatory inputs to corticospinal neurons during the warning period of various reaction time tasks. Using two separate methods (H-reflex conditioning and directional effects of TMS), we show that a specific set of excitatory inputs to corticospinal neurons are suppressed during motor preparation, while another set of inputs remain unaffected. To probe the behavioral relevance of this suppression, we examined whether the strength of the selective preparatory inhibition in each trial was related to reaction time. Surprisingly, the greater the amount of selective preparatory inhibition, the faster the reaction time was. This suggests that the inhibition of inputs to corticospinal neurons is not involved in preventing the release of movement but may in fact facilitate rapid reactions. Thus, selective suppression of a specific set of motor cortical neurons may be a key aspect of successful movement preparation.

SIGNIFICANCE STATEMENT Movement preparation evokes substantial activity in the motor cortex despite no apparent movement. One explanation for the lack of movement is that motor cortical output in this period is gated by an inhibitory mechanism. This notion was supported by previous noninvasive TMS studies of human motor cortex indicating a reduction of corticospinal excitability. On the contrary, our data support the idea that there is a coordinated balance of activity upstream of the corticospinal output neurons. This includes a suppression of specific local circuits that supports, rather than inhibits, the rapid generation of prepared movements. Thus, the selective suppression of local circuits appears to be an essential part of successful movement preparation instead of an external control mechanism.

Spontaneous activity and functional connectivity in the developing cerebellorubral system

The development of the cerebellar system depends in part on the emergence of functional connectivity in its input and output pathways. Characterization of spontaneous activity within these pathways provides insight into their functional status in early development. In the present study we recorded extracellular activity from the interpositus nucleus (IP) and its primary downstream target, the red nucleus (RN), in unanesthetized rats at postnatal days 8 (P8) and P12, a period of dramatic change in cerebellar circuitry. The two structures exhibited state-dependent activity patterns and age-related changes in rhythmicity and overall firing rate. Importantly, sensory feedback (i.e., reafference) from myoclonic twitches (spontaneous, self-generated movements that are produced exclusively during active sleep) drove neural activity in the IP and RN at both ages. Additionally, anatomic tracing confirmed the presence of cerebellorubral connections as early as P8. Finally, inactivation of the IP and adjacent nuclei using the GABAA receptor agonist muscimol caused a substantial decrease in neural activity in the contralateral RN at both ages, as well as the disappearance of rhythmicity; twitch-related activity in the RN, however, was preserved after IP inactivation, indicating that twitch-related reafference activates the two structures in parallel. Overall, the present findings point to the contributions of sleep-related spontaneous activity to the development of cerebellar networks.

Corticospinal Excitability of Trunk Muscles during Different Postural Tasks

Evidence suggests that the primary motor cortex (M1) is involved in both voluntary, goal-directed movements and in postural control. Trunk muscles are involved in both tasks, however, the extent to which M1 controls these muscles in trunk flexion/extension (voluntary movement) and in rapid shoulder flexion (postural control) remains unclear. The purpose of this study was to investigate this question by examining excitability of corticospinal inputs to trunk muscles during voluntary and postural tasks. Twenty healthy adults participated. Transcranial magnetic stimulation was delivered to the M1 to examine motor evoked potentials (MEPs) in the trunk muscles (erector spinae (ES) and rectus abdominis (RA)) during dynamic shoulder flexion (DSF), static shoulder flexion (SSF), and static trunk extension (STE). The level of background muscle activity in the ES muscles was matched across tasks. MEP amplitudes in ES were significantly larger in DSF than in SSF or in STE; however, this was not observed for RA. Further, there were no differences in levels of muscle activity in RA between tasks. Our findings reveal that corticospinal excitability of the ES muscles appears greater during dynamic anticipatory posture-related adjustments than during static tasks requiring postural (SSF) and goal-directed voluntary (STE) activity. These results suggest that task-oriented rehabilitation of trunk muscles should be considered for optimal transfer of therapeutic effect to function.

Corticomotor control of lumbar multifidus muscles is impaired in chronic low back pain: concurrent evidence from ultrasound imaging and double-pulse transcranial magnetic stimulation

Chronic low back pain (CLBP) is often associated with impaired control of deep trunk muscles and reorganization of the primary motor areas (M1). Precisely, functional changes of the lumbar multifidus muscles (MF) involved in spine stability may be of special interest in rehabilitation. Therefore, we tested MF corticomotor control using double transcranial magnetic stimulation (TMS) paradigms for the first time in this muscle and examined its link with MF volitional activation. Eleven individuals with lateralized CLBP and 13 pain-free participants were recruited. Ultrasound imaging enabled measurement of MF volitional isometric contraction in prone lying. TMS of MF M1 area was used to test hemispheric excitability and mechanisms in relation to motor programming, i.e., active motor threshold (AMT), amplitude of motor-evoked potentials and short-interval intracortical inhibition (SICI) and facilitation (SICF). In CLBP, SICI level was lower in the left hemisphere and MF volitional contraction was not related to AMT (M1 excitability), conversely to what was observed in the pain-free group. No other between-group difference was detected. These original findings support a plasticity of cortical maps controlling paravertebral muscles and likely including a different motor strategy for the control of MF. Changes of M1 function may thus underlie impaired motor control of lumbopelvic spine and pain persistence in CLBP.

Assessment of corticospinal excitability after traumatic spinal cord injury using MEP recruitment curves: a preliminary TMS study

STUDY DESIGN

Transcranial magnetic stimulation study.

OBJECTIVES

To further investigate the corticospinal excitability changes after spinal cord injury (SCI), as assessed by means of transcranial magnetic stimulation (TMS).

SETTING

Merano (Italy) and Salzburg (Austria).

METHODS

We studied resting motor threshold (RMT), motor evoked potential (MEP) amplitude and recruitment curve in five subjects with good recovery after traumatic incomplete cervical SCI.

RESULTS

RMT did not differ significantly between patients and controls, whereas the slope of MEP recruitment curve was significantly increased in the patients.

CONCLUSION

This abnormal finding may represent an adaptive response after SCI. The impaired ability of the motor cortex to generate proper voluntary movement may be compensated by increasing spinal excitability. The easily performed measurement of MEP recruitment curve may provide a useful additional tool to improve the assessment and monitoring of motor cortical function in subjects with SCI. Increasing our knowledge of the corticospinal excitability changes in the functional recovery after SCI may also support the development of effective therapeutic strategies.

Noninvasive neurostimulation in chronic stroke: a double-blind randomized sham-controlled testing of clinical and corticomotor effects

BACKGROUND

Repetitive peripheral magnetic stimulation (RPMS) is a painless and noninvasive method to produce afferents via the depolarization of the peripheral nervous system. A few studies tested RPMS after-effects on cerebral plasticity and motor recovery in stroke individuals, but evidences remain limited.

OBJECTIVES

This study aimed to explore whether RPMS could mediate improvements in corticomotor and clinical outcomes associated with ankle impairments in chronic stroke.

METHODS

Eighteen subjects with chronic stroke were randomly allocated to RPMS or sham group and compared to 14 healthy subjects. Stimulation was applied over the paretic tibialis anterior (TA). Ankle impairments on the paretic side and ipsilesional TA cortical motor representation were tested clinically and by transcranial magnetic stimulation (TMS), respectively.

RESULTS

In the RPMS group, ankle dorsiflexion mobility and maximal isometric strength increased and resistance to plantar flexor stretch decreased. The magnitude of change seemed to be related to cortical and corticospinal integrity. Sham stimulation yielded no effect. Changes in TMS outcome and their relationships with clinical improvements were limited.

CONCLUSIONS

RPMS improved ankle impairments in chronic stroke likely by a dynamic influence of sensory inputs on synaptic plasticity. The neurophysiological mechanisms potentially underlying the clinical effects are unclear. More studies are warranted to test the spinal and hemispheric changes responsible for the clinical improvements with emphasis on circuits spared by the lesion.

Transpinal and transcortical stimulation alter corticospinal excitability and increase spinal output

The objective of this study was to assess changes in corticospinal excitability and spinal output following noninvasive transpinal and transcortical stimulation in humans. The size of the motor evoked potentials (MEPs), induced by transcranial magnetic stimulation (TMS) and recorded from the right plantar flexor and extensor muscles, was assessed following transcutaneous electric stimulation of the spine (tsESS) over the thoracolumbar region at conditioning-test (C-T) intervals that ranged from negative 50 to positive 50 ms. The size of the transpinal evoked potentials (TEPs), induced by tsESS and recorded from the right and left plantar flexor and extensor muscles, was assessed following TMS over the left primary motor cortex at 0.7 and at 1.1× MEP resting threshold at C-T intervals that ranged from negative 50 to positive 50 ms. The recruitment curves of MEPs and TEPs had a similar shape, and statistically significant differences between the sigmoid function parameters of MEPs and TEPs were not found. Anodal tsESS resulted in early MEP depression followed by long-latency MEP facilitation of both ankle plantar flexors and extensors. TEPs of ankle plantar flexors and extensors were increased regardless TMS intensity level. Subthreshold and suprathreshold TMS induced short-latency TEP facilitation that was larger in the TEPs ipsilateral to TMS. Noninvasive transpinal stimulation affected ipsilateral and contralateral actions of corticospinal neurons, while corticocortical and corticospinal descending volleys increased TEPs in both limbs. Transpinal and transcortical stimulation is a noninvasive neuromodulation method that alters corticospinal excitability and increases motor output of multiple spinal segments in humans.

Noninvasive and painless magnetic stimulation of nerves improved brain motor function and mobility in a cerebral palsy case

Motor deficits in cerebral palsy disturb functional independence. This study tested whether noninvasive and painless repetitive peripheral magnetic stimulation could improve motor function in a 7-year-old boy with spastic hemiparetic cerebral palsy. Stimulation was applied over different nerves of the lower limbs for 5 sessions. We measured the concurrent aftereffects of this intervention on ankle motor control, gait (walking velocity, stride length, cadence, cycle duration), and function of brain motor pathways. We observed a decrease of ankle plantar flexors resistance to stretch, an increase of active dorsiflexion range of movement, and improvements of corticospinal control of ankle dorsiflexors. Joint mobility changes were still present 15 days after the end of stimulation, when all gait parameters were also improved. Resistance to stretch was still lower than prestimulation values 45 days after the end of stimulation. This case illustrates the sustained effects of repetitive peripheral magnetic stimulation on brain plasticity, motor function, and gait. It suggests a potential impact for physical rehabilitation in cerebral palsy.

Brain control of volitional ankle tasks in people with chronic stroke and in healthy individuals

This study explored the relationships between motor cortical control of ankle dorsiflexors and clinical impairments of volitional ankle dorsiflexion in people with chronic stroke. Eighteen persons with stroke and 14 controls were evaluated. Clinical tools were used to assess ankle dorsiflexion amplitude and isometric strength. Transcranial magnetic stimulation (TMS) of the primary motor cortex (M1) tested the functional integrity of cortical circuits controlling the tibialis anterior (TA). All clinical scores and most TMS outcomes were impaired in people with chronic stroke. The lower clinical scores were related to the reduction of the strength of corticospinal projections onto spinal motoneurons. Concurrent TMS and clinical testing in chronic stroke provided original data demonstrating relationships between the integrity of cortical and corticospinal components of TA motor control and volitional ankle tasks. Our study proposes that volitional ankle mobilization in chronic stroke may be explained by the residual abnormal M1 circuits which may be responsive for rehabilitation intervention. This should be confirmed in longitudinal studies with larger samples to determine whether TMS outcomes associated with lower limb muscles are predictive of clinical changes or vice versa.

Tracking post-error adaptation in the motor system by transcranial magnetic stimulation

The commission of an error triggers cognitive control processes dedicated to error correction and prevention. Post-error adjustments leading to response slowing following an error ("post-error slowing"; PES) might be driven by changes in excitability of the motor regions and the corticospinal tract (CST). The time-course of such excitability modulations of the CST leading to PES is largely unknown. To track these presumed excitability changes after an error, single pulse transcranial magnetic stimulation (TMS) was applied to the motor cortex ipsilateral to the responding hand, while participants were performing an Eriksen flanker task. A robotic arm with a movement compensation system was used to maintain the TMS coil in the correct position during the experiment. Magnetic pulses were delivered over the primary motor cortex ipsilateral to the active hand at different intervals (150, 300, 450 ms) after correct and erroneous responses, and the motor-evoked potentials (MEP) of the first dorsal interosseous muscle (FDI) contralateral to the stimulated hemisphere were recorded. MEP amplitude was increased 450 ms after the error. Two additional experiments showed that this increase was neither associated to the correction of the erroneous responses nor to the characteristics of the motor command. To the extent to which the excitability of the motor cortex ipsi- and contralateral to the response hand are inversely related, these results suggest a decrease in the excitability of the active motor cortex after an erroneous response. This modulation of the activity of the CST serves to prevent further premature and erroneous responses. At a more general level, the study shows the power of the TMS technique for the exploration of the temporal evolution of post-error adjustments within the motor system.

Enhanced corticospinal excitability with physiologically heightened sympathetic nerve activity

Corticospinal excitability is modulated differently with norepinephrine and dopamine agonists, although both monoamines are released with heightened sympathetic nerve activity. The purpose of this study was to investigate the influence of physiological heightening of sympathetic nerve activity on corticospinal excitability in healthy humans. Subjects were divided into control and experimental groups. In each participant, motor-evoked potentials (MEPs) were measured from the resting first dorsal interosseous muscle of the right hand with transcranial magnetic stimulation (TMS) in two trials separated by 1 h. In the experimental group, sympathetic nerve activity was physiologically heightened during the second trial by applying lower body negative pressure (LBNP). In the control group, sympathetic nerve activity was not altered between the two trials. MEP peak-to-peak amplitude increased from trial 1 to trial 2 in the experimental group only. This increase was evident at a TMS intensity of 130% resting motor threshold and higher. It was concluded that physiological heightening of sympathetic nerve activity with LBNP enhances corticospinal excitability.

Plasticity of corticospinal neural control after locomotor training in human spinal cord injury

Spinal lesions substantially impair ambulation, occur generally in young and otherwise healthy individuals, and result in devastating effects on quality of life. Restoration of locomotion after damage to the spinal cord is challenging because axons of the damaged neurons do not regenerate spontaneously. Body-weight-supported treadmill training (BWSTT) is a therapeutic approach in which a person with a spinal cord injury (SCI) steps on a motorized treadmill while some body weight is removed through an upper body harness. BWSTT improves temporal gait parameters, muscle activation patterns, and clinical outcome measures in persons with SCI. These changes are likely the result of reorganization that occurs simultaneously in supraspinal and spinal cord neural circuits. This paper will focus on the cortical control of human locomotion and motor output, spinal reflex circuits, and spinal interneuronal circuits and how corticospinal control is reorganized after locomotor training in people with SCI. Based on neurophysiological studies, it is apparent that corticospinal plasticity is involved in restoration of locomotion after training. However, the neural mechanisms underlying restoration of lost voluntary motor function are not well understood and translational neuroscience research is needed so patient-orientated rehabilitation protocols to be developed.

Age-related differences in corticospinal excitability during a Go/NoGo task

Age-related slowing of reaction times (RTs) is well documented but whether the phenomenon reflects deficits in movement preparation and/or response generation processes is unclear. To gain further insight into this issue, transcranial magnetic stimulation (TMS) was used to investigate motor cortex (M1) excitability and short-interval intracortical inhibitory (SICI) processes during a Go/NoGo RT task in younger and older adults. Single- and paired-pulse TMS was delivered over the left M1 during preparation and response generation periods in a right-hand muscle. Younger adults had shorter RTs and a larger increase in corticospinal excitability at response generation period than older adults. SICI modulation for both groups showed a large reduction in inhibition immediately prior to EMG onset. These findings indicate age-related differences in corticospinal excitability during the response generation stage of sensorimotor information processing.

Involvement of the corticospinal tract in the control of human gait

Given the inherent mechanical complexity of human bipedal locomotion, and that complete spinal cord lesions in human leads to paralysis with no recovery of gait, it is often suggested that the corticospinal tract (CST) has a more predominant role in the control of walking in humans than in other animals. However, what do we actually know about the contribution of the CST to the control of gait? This chapter will provide an overview of this topic based on the premise that a better understanding of the role of the CST in gait will be essential for the design of evidence-based approaches to rehabilitation therapy, which will enhance gait ability and recovery in patients with lesions to the central nervous system (CNS). We review evidence for the involvement of the primary motor cortex and the CST during normal and perturbed walking and during gait adaptation. We will also discuss knowledge on the CST that has been gained from studies involving CNS lesions, with a particular focus on recent data acquired in people with spinal cord injury.

Control of wrist position and muscle relaxation by shifting spatial frames of reference for motoneuronal recruitment: possible involvement of corticospinal pathways

It has previously been established that muscles become active in response to deviations from a threshold (referent) position of the body or its segments, and that intentional motor actions result from central shifts in the referent position. We tested the hypothesis that corticospinal pathways are involved in threshold position control during intentional changes in the wrist position in humans. Subjects moved the wrist from an initial extended to a final flexed position (and vice versa). Passive wrist muscle forces were compensated with a torque motor such that wrist muscle activity was equalized at the two positions. It appeared that motoneuronal excitability tested by brief muscle stretches was also similar at these positions. Responses to mechanical perturbations before and after movement showed that the wrist threshold position was reset when voluntary changes in the joint angle were made. Although the excitability of motoneurons was similar at the two positions, the same transcranial magnetic stimulus (TMS) elicited a wrist extensor jerk in the extension position and a flexor jerk in the flexion position. Extensor motor-evoked potentials (MEPs) elicited by TMS at the wrist extension position were substantially bigger compared to those at the flexion position and vice versa for flexor MEPs. MEPs were substantially reduced when subjects fully relaxed wrist muscles and the wrist was held passively in each position. Results suggest that the corticospinal pathway, possibly with other descending pathways, participates in threshold position control, a process that pre-determines the spatial frame of reference in which the neuromuscular periphery is constrained to work. This control strategy would underlie not only intentional changes in the joint position, but also muscle relaxation. The notion that the motor cortex may control motor actions by shifting spatial frames of reference opens a new avenue in the analysis and understanding of brain function.

Corticospinal contribution to arm muscle activity during human walking

When we walk, our arm muscles show rhythmic activity suggesting that the central nervous system contributes to the swing of the arms. The purpose of the present study was to investigate whether corticospinal drive plays a role in the control of arm muscle activity during human walking. Motor evoked potentials (MEPs) elicited in the posterior deltoid muscle (PD) by transcranial magnetic stimulation (TMS) were modulated during the gait cycle in parallel with changes in the background EMG activity. There was no significant difference in the size of the MEPs at a comparable level of background EMG during walking and during static PD contraction. Short latency intracortical inhibition (SICI; 2 ms interval) studied by paired-pulse TMS was diminished during bursts of PD EMG activity. This could not be explained only by changes in background EMG activity and/or control MEP size, since SICI showed no correlation to the level of background EMG activity during static PD contraction. Finally, TMS at intensity below the threshold for activation of corticospinal tract fibres elicited a suppression of the PD EMG activity during walking. Since TMS at this intensity is likely to only activate intracortical inhibitory interneurones, the suppression is in all likelihood caused by removal of a corticospinal contribution to the ongoing EMG activity. The data thus suggest that the motor cortex makes an active contribution, through the corticospinal tract, to the ongoing EMG activity in arm muscles during walking.

Supra-spinal circuits shape inhibitory postural adjustments anticipating voluntary index-finger flexion

We previously observed that index finger flexion, with the prone hand, is preceded by an inhibitory postural adjustment (iAPA) in the Biceps Brachii (BB) electromyogram (EMG). The aim of this work is to locate the iAPA origin by testing the changes in spinal and cortico-spinal excitability during its development. Subjects were asked to flex their index finger, at will after an acoustic signal, while surface EMGs from Flexor Digitorum Superficialis (prime mover) and from BB (postural muscle) were recorded. In each experiment, tendon (T) reflexes in the resting BB and motor-evoked potentials elicited by magnetic cortical stimulation (CMEPs) in the resting, or in the slightly contracted BB, were collected both during the iAPA development and before the go signal. When BB was fully relaxed, T-reflexes falling in the iAPA temporal window were un-modulated, while CMEPs were significantly inhibited; this inhibition occurred, on average, 37 ms before the iAPA onset indicating that iAPA sets up and develops at supraspinal level. Changes in CMEPs' excitability clearly anticipated the iAPA onset (about 50 ms) also when cortico-spinal excitability was probed in the active BB, thus indicating that this effect is likely due to a de-recruitment of spinal motoneurones. Thus, (1) iAPA is associated with voluntary movement even when postural muscles are at rest, (2) during iAPA spinal motoneurones are more likely disfacilitated than inhibited, and (3) the iAPA timing is regulated similarly to prime mover activation. All together these results suggest that iAPAs is fully sustained by inhibitory circuits located in the supra-spinal centres.

Cortical involvement in anticipatory postural reactions in man

All movements are accompanied by postural reactions which ensure that the balance of the body is maintained. It has not been resolved that to what extent the primary motor cortex and corticospinal tract are involved in the control of these reactions. Here, we investigated the contribution of the corticospinal tract to the activation of the soleus (SOL) muscle in standing human subjects (n=10) in relation to voluntary heel raise, anticipatory postural activation of the soleus muscle when the subject pulled a handle and to reflex activation of the soleus muscle when the subject was suddenly pulled forward by an external perturbation. SOL motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) increased significantly in relation to rest -75 ms prior to the onset of EMG in the heel-raise and handle-pull tasks. The short-latency facilitation of the soleus H-reflex evoked by TMS increased similarly, suggesting that the increased MEP size prior to movement was caused at least partly by increased excitability of corticospinal tract cells with monosynaptic projections to SOL motoneurones. Changes in spinal motoneuronal excitability could be ruled out since there was no significant increase of the SOL H-reflex until immediately prior to EMG onset for any of the tasks. Tibialis anterior MEPs were unaltered prior to the onset of SOL EMG activity in the handle-pull task, suggesting that the MEP facilitation was specific for the SOL muscle. No significant increase of the MEPs was observed prior to EMG onset for the external perturbation. These data suggest that the primary motor cortex is involved in activating the SOL muscle as part of an anticipatory postural reaction.

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.

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.

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.

Reduction of intracortical inhibition in soleus muscle during postural activity

Short-interval intracortical inhibition (SICI) decreases during voluntary contraction of the target muscle. It is unknown whether this effect also occurs with postural contractions. We have compared the effects of voluntary and postural contractions on SICI in the soleus (SOL) muscle. We applied transcranial magnetic stimuli (TMS) in subjects under three tasks: sitting at rest (Rest), sitting while activating the SOL muscle (Voluntary), or standing quietly (Postural). In control trials, we applied suprathreshold TMS to obtain unconditioned motor-evoked potentials (MEPs). In test trials, the same TMS was preceded by a subthreshold TMS at different interstimulus intervals (ISIs), to obtain a conditioned MEP. SICI and intracortical facilitation (ICF) were expressed as the decrease or increase in MEP size relative to unconditioned MEPs. There was significant effect of task in mean SICI or mean ICF in SOL. Mean SICI in SOL was 52% in Rest and decreased to 21% in Voluntary and 15% in Postural. Mean ICF in SOL was 132% and decreased to 113% in Voluntary and to 108% in Postural. Mean SICI in SOL was not different in Voluntary and Postural tasks. There was no effect of task in mean SICI or mean ICF in TA. Our results indicate that decrease of SICI with muscle contraction occurs to a similar extent with tonic voluntary and postural activation, suggesting that those contractions require a similar type of cortical involvement. However, it cannot be excluded that some part of the SICI reduction with muscle contraction depends on changes in segmental excitability.