Disinhibition of upper limb motor area by voluntary contraction of the lower limb muscle

It is well known that monosynaptic spinal reflexes and motor evoked potentials following transcranial magnetic stimulation (TMS) are reinforced during phasic and intensive voluntary contraction in the remote segment (remote effect). However, the remote effect on the cortical silent period (CSP) is less known. The purpose of the present study is to determine to what extent the CSP in the intrinsic hand muscle following TMS is modified by voluntary ankle dorsiflexion and to elucidate the origin of the modulation of CSP by the remote effect. CSP was recorded in the right first dorsal interosseous while subjects performed phasic dorsiflexion in the ipsilateral side under self-paced and reaction-time conditions. Modulation of the peripherally-induced silent period (PSP) induced by electrical stimulation of the ulnar nerve was also investigated under the same conditions. In addition, modulation of the CSP was investigated during ischemic nerve block of the lower limb and during application of vibration to the tibialis anterior tendon. The duration of CSP was significantly shortened by phasic dorsiflexion, and the extent of shortening was proportional to dorsiflexion force. Shortening of the CSP duration was also observed during tonic dorsiflexion. In contrast, the PSP duration following ulnar nerve stimulation was not altered during phasic dorsiflexion. Furthermore, the remote effect on the CSP duration was seen during ischemic nerve block of the lower limb and the pre-movement period in the reaction-time paradigm, but shortening of the CSP was not observed during tendon vibration. These findings suggest that phasic muscle contraction in the remote segment results in a decrease in intracortical inhibitory pathways to the corticospinal tract innervating the muscle involved in reflex testing and that the remote effect on the CSP is predominantly cortical in origin.     

Characterisation of human limb movements by accelerometry

The paper discusses the use of accelerometry as an alternative to electromyography or subjective judgement in the assessment of functional disability. The apparatus described is an accelerometer with an f.m.-f.m. telemetry transmitter, coupled to an f.m. receiver, signal conditioner, subcarrier demodulator and twin integrator to generate positional information from acceleartion. The capabilities and limitations of accelerometry are discussed, typical recordings are presented, and interpretation of recordings is described. It is concluded that the method is well worth pursuing.     

Arm movements evoked by electrical stimulation in the motor cortex of monkeys

Electrical stimulation of the motor cortex in monkeys can evoke complex, multijoint movements including movements of the arm and hand. In this study, we examined these movements in detail and tested whether they showed adaptability to differing circumstances such as to a weight added to the hand. Electrical microstimulation was applied to motor cortex using pulse trains of 500-ms duration (matching the approximate duration of a reach). Arm movement was measured using a high-resolution three-dimensional tracking system. Movement latencies averaged 80.2 ms. Speed profiles were typically smooth and bell-shaped, and the peak speed covaried with movement distance. Stimulation generally evoked a specific final hand position. The convergence of the hand from disparate starting positions to a narrow range of final positions was statistically significant for every site tested (91/91). When a weight was fixed to the hand, for some stimulation sites (74%), the evoked movement appeared to compensate for the weight in that the hand was lifted to a similar final location. For other stimulation sites (26%), the weight caused a significant reduction in final hand height. For about one-half of the sites (54%), the variation in movement of each joint appeared to compensate for the variation in the other joints in a manner that stabilized the hand in a restricted region of space. These findings suggest that at least some of the stimulation-evoked movements reflect relatively high-level, adaptable motor plans.     

Classically conditioned limb reflexes reinforced by motor cortex stimulation

This study measures the latencies of, and the effect of the reduction of the interreinforcement time interval upon, conditional limb reflexes reinforced to an auditory conditional stimulus with movement evoking electrical stimulation of the motor-sensory cortex as the unconditional stimulus. Conditioned head movements were obtained in all, while limb movements developed in only 2, of 3 cats following 4.6 min reinforcement intervals. One-min reinforcement intervals did not abolish these limb responses which are here shown to have a latency significantly in excess of 1 sec. The conditional limb responses of this study are identified as a classical, as contrasted with the instrumental variant of other studies. The classical limb response is thought to develop if there is: (a) an absence of head movement induced extensor tonus; (b) a motor-sensory cortex stimulus adequate to evoke limb movement; and, (c) a period of conditional stimulus isolation ample to permit the observation of these long latency responses.     

Motion quality evaluation of upper limb target-reaching movements

Fitts' Law was extended in the polar coordinate system, and a set of indices for human motion evaluation is proposed. In this paper, the index of difficulty and the index of performance are introduced as the general indices for the quality measure of plane target-to-target movement. As an example, the target-reaching movement of the upper limb, which is a basic functional action of upper limbs in the activities of daily living, was experimentally investigated. Five healthy subjects were asked to perform six target-reaching tasks with different indices of difficulty. All movements were recorded using a Vicon motion analysis system. The movement quality was measured using these evaluation indices.     

Mapping of direction and muscle representation in the human primary motor cortex controlling thumb movements

Larger body parts are somatotopically represented in the primary motor cortex (M1), while smaller body parts, such as the fingers, have partially overlapping representations. The principles that govern the overlapping organization of M1 remain unclear. We used transcranial magnetic stimulation (TMS) to examine the cortical encoding of thumb movements in M1 of healthy humans. We performed M1 mapping of the probability of inducing a thumb movement in a particular direction and used low intensity TMS to disturb a voluntary thumb movement in the same direction during a reaction time task. With both techniques we found spatially segregated representations of the direction of TMS-induced thumb movements, thumb flexion and extension being best separated. Furthermore, the cortical regions corresponding to activation of a thumb muscle differ, depending on whether the muscle functions as agonist or as antagonist for flexion or extension. In addition, we found in the reaction time experiment that the direction of a movement is processed in M1 before the muscles participating in it are activated. It thus appears that one of the organizing principles for the human corticospinal motor system is based on a spatially segregated representation of movement directions and that the representation of individual somatic structures, such as the hand muscles, overlap.     

Disinhibition in the monkey prefrontal cortex, by injecting bicuculline, induces forelimb movements learned in a GO/NO-GO task

Local injection of a GABA (gamma-aminobutyric acid) antagonist, bicuculline, into Brodmann's area 8 (Walker's areas 8A and 45) of the monkey prefrontal cortex induced movements of the contralateral forelimb in 4 monkeys that were well-trained to perform a two-choice visual discrimination GO/NO-GO task by appropriate release of the hand from a lever. But the same procedures of injection in two naive monkeys did not. These results indicate that, during learning of the task, a new structure responsible for learned movement of the forelimb was made in area 8 and was disinhibited by bicuculline and, furthermore, they suggest that inhibitory GABA neurons suppress tonically efferent neurons associated with the initiation of movement.     

在线调整对到达运动中人体上肢刚性特性的影响

目的研究在线调整与运动中人体上肢刚性值之间的关系,提高人体刚性值测量的精度以及评判在线调整能力。方法设计5类不同类型的上肢平面到达运动,通过施加快速扰动的方法,对运动过程的前期、前中期、中期、中后期和后期共5个位置的刚性值进行测量,探讨刚性值在受到在线调整影响时的变化规律,以及刚性幅值与运动精度之间的关系。结果运动过程中刚性一直变化,而且刚性值的变化影响了最终的运动误差。运动过程中在线调整的发生会引起刚性幅值的改变,尤其是运动后期的刚性幅值,但是目前没有发现这一变化与在线调整发生的时间或者内容存在密切的联系。结论在线调整在到达运动中扮演了重要的角色。考虑到在线调整的发生会引起刚性幅值的变化,在临床上,通过对患者在特定实验中刚性幅值的测量,结合其他医学诊断方法,可以更精确地表明患者当前在线调整功能的状况。     

Modification of the human motor cortex by associative stimulation

Manipulation of afferent input is capable of inducing reorganisation of the motor cortex. For example, following 1 h of paired electrical stimulation to the motor point of two hand muscles (associative stimulation) the excitability of the corticospinal projection to the stimulated muscles is increased. Here we investigated the mechanisms responsible for such change using transcranial magnetic stimulation (TMS). Cortical excitability changes were investigated by measuring motor evoked potentials (MEPs), intracortical inhibition (ICI), intracortical facilitation (ICF), and short-interval intracortical facilitation (SICF). Following 1 h of associative stimulation MEP amplitudes in the stimulated muscles significantly increased. Additionally, there was a significant increase in ICF and of SICF at interstimulus intervals in the range of 2.3–3.3 ms. There was no significant change in ICI. These findings confirm previous observations that a 1-h period of associative stimulation can increase the excitability of the cortical projection to stimulated muscles. Additionally, these results suggest that the observed modifications of excitability are due to changes in intracortical excitatory circuits.     

Perturbed upper limb movements cause short-latency postural responses in trunk muscles

Addition of a load to a moving upper limb produces a perturbation of the trunk due to transmission of mechanical forces. This experiment investigated the postural response of the trunk muscles in relation to unexpected limb loading. Subjects performed rapid, bilateral shoulder flexion in response to a stimulus. In one third of trials, an unexpected load was added bilaterally to the upper limbs in the first third of the movement. Trunk muscle electromyography, intra-abdominal pressure and upper limb and trunk motion were measured. A short-latency response of the erector spinae and transversus abdominis muscles occurred approximately 50 ms after the onset of the limb perturbation that resulted from addition of the load early in the movement and was coincident with the onset of the observed perturbation at the trunk. The results provide evidence of initiation of a complex postural response of the trunk muscles that is consistent with mediation by afferent input from a site distant to the lumbar spine, which may include afferents of the upper limb.     

Role of the human motor cortex in rapid motor learning

Recent studies suggest that the human primary motor cortex (M1) is involved in motor learning, but the nature of that involvement is not clear. Here, learning-related changes in M1 excitability were studied with transcranial magnetic stimulation (TMS) while na subjects practiced either a ballistic or a ramp pinch task to the 0.5-Hz beat of a metronome. Subjects rapidly learned to optimize ballistic contractions as indicated by a significant increase in peak pinch acceleration and peak force after the 60-min practice epoch. The increase in force and acceleration was associated with an increase in motor evoked potential (MEP) amplitude in a muscle involved in the training (flexor policis brevis) but not in a muscle unrelated to the task (abductor digiti minimi). MEPs returned to their baseline amplitude after subjects had acquired the new skill, whereas no practice-induced changes in MEP amplitude were observed after subjects had overlearned the task, or after practicing slow ramp pinches. Since the changes in MEP amplitude were observed only after TMS of M1 but not after direct stimulation of the corticospinal tract, these findings indicate task- and effector-specific involvement of human M1 in rapid motor learning.     

The association of upper airway resistance with periodic limb movements

STUDY OBJECTIVES: We hypothesized that the upper airway resistance syndrome (UARS), the component event being a respiratory effort related arousal (RERA), and periodic limb movements in sleep (PLMS), the component event being repetitive, stereotyped extremity movements occurring in a periodic fashion, were associated in certain patients. DESIGN: Invasive polysomnography using Pes and full facemask pneumotachography was used to identify RERA's in patients. Periodic limb movements (PLM) were scored according to standard criteria and as associated with RERA if the movement occurred between the Pes nadir and the onset of the arousal. SETTING: A university hospital Sleep Disorders Laboratory. PARTICIPANTS: Patients consecutively diagnosed with PLMS in our sleep disorders laboratory over a 1 year period. INTERVENTIONS: None. MEASUREMENTS AND RESULTS: Fourteen of twenty patients demonstrated UARS in addition to PLMS (70%). In those 14, 63% of RERAs were associated with a PLM (mean = 51.7 + 36.2 PLM/RERAs per study vs 5.6 + 6.3 PLM/RERAs per study if the association were random). Patients with UARS had more arousals with their PLMs (P = 0.0006). CONCLUSIONS: An association exists between PLMS and UARS on both a group level and an event level. A high percentage of PLM with arousals correlated with breathing events due to increased effort in UARS; this may be of clinical utility in the management of PLMS patients.     

Voluntary rhythmical movement is reset by stimulating the motor cortex

Using six normal subjects, we mapped the best location for magnetic cortical stimulation to reset the phase of a voluntary alternating movement of the right wrist made against three different torques (0.26 N m extension, 0 and 0.09 N m flexion torque) at the subjects' preferred rate. We used nett resetting as a measure of phase resetting, based upon the relative amplitudes of the averages of the stimulated and a phase-locked control position record. Nine sites covering a 5 cm square region of the contralateral cortex were systematically stimulated. All the subjects showed evidence of resetting in response to magnetic stimulation over one or more cortical sites during movements made against the extension torque and all subjects demonstrated higher levels of nett resetting under these conditions than in response to similar cortical stimulation during unloaded movements. The best cortical sites for inducing resetting were the same as those from which the largest short-latency responses were evoked in the contralateral forearm flexor and extensor muscles, i.e. the motor cortex. At the cortical sites where magnetic stimulation did induce resetting, the initial electromyographic (EMG) effects consisted of a short-latency excitation followed by a period of inhibition. This silent period was followed by a short burst of excitation often occurring simultaneously in the wrist flexor and extensor muscles, and only thereafter by the return of rhythmical alternating EMG activity characteristic of the wrist movement. The latency to the first rhythmical EMG peak following the stimulus was closely related to the period of the subject's prestimulus movement.     

Facilitatory I wave interaction in proximal arm and lower limb muscle representations of the human motor cortex

Transcranial magnetic stimulation (TMS) of the human motor cortex elicits direct and indirect (I) waves in the corticospinal tract. Facilitatory I wave interaction has been demonstrated with a suprathreshold first stimulus (S1) followed by a subthreshold to threshold second stimulus (S2). Intracortical inhibition (ICI) and intracortical facilitation (ICF) can be studied by another paired TMS paradigm with a subthreshold conditioning stimulus (CS) followed by a suprathreshold test stimulus. Facilitatory I wave interaction in motor representations other than the hand area and its relationship to ICI and ICF has not been studied. We studied I wave interaction, ICI and ICF in an intrinsic hand muscle (abductor pollicis brevis, APB), in a proximal arm muscle (biceps brachii, BB) and in a lower limb muscle (tibialis anterior, TA) in 11 normal subjects. I wave facilitation was studied by paired TMS at 24 interstimulus intervals (ISIs) from 0.5 to 5.1 ms. For APB and TA, facilitation occurred in three distinct peaks at ISIs of 0.9-1.7, 2. 5-3.5, and 4.1-5.1 ms. For BB, facilitation was significant for the first two peaks. The latencies of the peaks were similar among different muscles, but the magnitude of facilitation was much greater for APB and TA compared with BB. For all three muscles, changing the S2 to transcranial electrical stimulation (TES) resulted in much less facilitation of the first peak. For APB, there was significant I wave facilitation with S2 at 72% motor threshold (MT). The same stimulus used as the CS did not elicit ICF at ISI of 15 ms, suggesting that the threshold for eliciting I wave facilitation is lower than that for ICF. For BB and TA, there was no I wave facilitation with S2 at 90% of APB MT, and the same stimulus used as CS led to ICI. Thus in BB and TA the threshold for eliciting ICI is lower than that for I wave facilitation. We conclude that the circuits that mediate I wave interactions are present in the proximal arm and lower limb representations of the motor cortex. I wave facilitation occurs predominately in the cortex and may be primarily related to the monosynaptic corticomotoneuronal (CM) system. The reduced I wave facilitation for BB compared with APB and TA may be related to less extensive CM projection and involvement of other polysynaptic descending pathways. I wave facilitation, ICI, and ICF appears to be mediated by different neuronal circuits.     

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.     

Reliability and validity of neuroelectric source imaging in primary somatosensory cortex of human upper limb amputees

The present study investigated the test-retest reliability of EEG source localizations of somatosensory evoked potentials (SEPs) in human upper limb amputees over a long time frame (several months) and examined the validity of source reconstruction. In two sessions spaced several months apart five unilateral upper limb amputees were stimulated at the first and fifth digit of the intact hand and at the left and right lower corner of the mouth. To examine the validity of the results of the neuroelectric source reconstruction a comparison with neuromagnetic source localization was performed for two subjects. The source localizations of the SEP components were found to be highly reproducible: the mean standard deviation of the dipole locations was 8.80 mm in the x-, 7.00 mm in the y- and 4.15 mm in the z-direction. The match of the comparison of EEG and MEG data was in the range of one centimeter. These results support the use of multi-electrode EEG recordings combined with MRI as an adequate method for the investigation of the functional organization of the somatosensory cortex in upper limb amputees and suggest high stability of cortical reorganization in these subjects.     

Influence of cold shivering on fine motor control in the upper limb

The aim of the investigation was to determine the effects of cold shivering on the accuracy of force output in distal, middle and proximal muscles of the upper limb. Tests of hand grip strength, elbow flexion and shoulder flexion (each done at 10% maximal voluntary contraction for 15 s) were done under three conditions: (1) thermoneutral air (27 °C), a condition of thermal comfort; (2) cold air (10 °C), a condition eliciting an increase in tonic muscle activity; (3) and cold air (10 °C) with a cold drink (8 °C), a condition that causes visible shivering. The averaged (root mean square) electromyogram (AEMG) and mean power frequency (MPF) were measured from proximal, middle and distal arm muscles during the tests and compared. The control of force output was highly effective at thermoneutral condition for all motor tasks. During the cold air condition, all muscles were tonically active but there was no effect on accuracy of test performance. However, AEMG increased ≈20% (P < 0.05) with respect to test performance in thermoneutral condition. During the cold air/cold drink condition, all muscles were shivering to a different extend. AEMG during test performance increased 30–150% in comparison to thermoneutral condition (P < 0.05). In this case, hand grip and elbow flexion were not adversely affected (these tests require middle and distal muscles) by cold shivering. However, the accuracy of performance of shoulder flexion was adversely affected. This is consistent with the fact that proximal muscles are more active during cold shivering.