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

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

Differential effect of muscle vibration on intracortical inhibitory circuits in humans

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

Dynamic changes in corticospinal excitability during motor imagery

 We investigated temporal changes in the amplitudes of motor-evoked potentials (MEPs) induced by transcranial magnetic stimulation over the left motor cortex during motor imagery. Nine subjects were instructed to imagine repetitive wrist flexion and extension movements at 1 Hz, in which the flexion timing was cued by a tone signal. Electromyographs (EMGs) were recorded from the first dorsal interosseous, flexor carpi radialis and extensor carpi radialis muscles of the right hand, and magnetic stimulation was delivered at 0, 250, 500 and 750 ms after the auditory cue. On average, the evoked EMG responses were larger in the flexor muscle during the phase of imagined flexion than during extension, whilst the opposite was true for the extensor muscle. There were no consistent changes in the amplitudes of MEPs in the intrinsic hand muscle (first dorsal interosseous). The EMG remained relaxed in all muscles and did not show any significant temporal changes during the test. The H-reflex in the flexor muscle was obtained in four subjects. There was no change in its amplitude during motor imagery. These observations lead us to suggest that motor imagery can have dynamic effects on the excitability of motor cortex similar to those seen during actual motor performance. Received: 23 July 1998 / Accepted: 26 October 1998     

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

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

Vibration-evoked reciprocal inhibition between human wrist muscles

Summary Reciprocal inhibition of the voluntarily contracting wrist extensor (extensor carpi radialis, ECR) evoked by proprioceptive afferent input from the flexor (flexor carpi radialis, FCR), was studied in healthy human subjects. Vibration of the FCR tendon was used to elicit Ia-dominated afferent discharge whilst inhibition of ECR was assessed as the reduction in asynchronous, on-going EMG. A small early phase of inhibition (I1) was evident in 25% of trials. The latency (ca. 25 ms) of this component suggested that it was mediated by an Ia oligosynaptic, possibly classical disynaptic, inhibitory pathway. A later and apparently separate phase of reduced activity (12, ca. 40 ms) was, however, far more consistently observed (96% of trials) and of greater magnitude. The 12 component was usually followed, some 20 ms later, by a phase of elevated activity (El, 72% trials). Reductions in simultaneously recorded net extensor torque commenced at about 60 ms following the onset of flexor tendon vibration, i.e. some 20 ms after the main I2 EMG component. These mechanical responses must have almost exclusively resulted from reciprocal inhibition of extensor EMG since vibration of the relaxed FCR evoked minimal excitatory flexor activity. The reflex pattern, in any individual subject, was relatively unaffected by altering the duration of the vibration train between one and nineteen cycles (125 Hz). This suggests that the entire response complex resulted largely from the initial afferent volley. The sizes of both the I1 and I2 reductions in ECR activity increased with increasing voluntary extensor contraction so that their depths remained constant proportions of background EMG. Very similar results were obtained when reciprocal inhibition of FCR was produced by vibration of the belly of ECR. Thus, reciprocal inhibition between wrist muscles is mainly expressed as a rather stereotyped, short duration reduction in EMG whose depth is determined by the pre-existing level of motor activity. Some functional implications of this form of reflex behaviour are discussed.     

Changes in Corticospinal Excitability in the Reactions of Forearm Muscles in Humans to Vibration

Transcranial magnetic stimulation was used to study changes in corticospinal excitability during vibration of the flexor and extensor muscles of the wrist in healthy humans. The ratios of muscle stimulation responses to activity levels in these muscles on contraction associated with vibration (the tonic vibratory reflex, TVR) and after vibration of antagonist muscles in isometric conditions (the antagonist vibratory reflex, AVR) were analyzed. The normalized muscle response in the wrist flexors was found to increase by 66% compared with threshold values in the direct vibratory response (TVR), by 75% in the relayed vibratory response (AVR), and by 18% on voluntary contraction. However, increases in the motor response in vibratory responses as compared with those on voluntary contraction did not reach significance, which contrasted with the responses in the forearm flexors. These results are discussed from the point of view that the motor cortex plays different roles in vibratory responses in the distal and proximal muscles.     

Cortical excitability and motor task in man: an investigation of the wrist extensor motor area

Task-related changes in the corticospinal excitation of the right extensor carpi radialis (ECR) muscle were investigated in 16 healthy human subjects. The subjects were asked to perform a tonic isometric wrist extension or to clench their hand around a manipulandum, thereby coactivating the antagonistic wrist muscles. At matched levels of background EMG in the ECR muscle, transcranial magnetic stimulation (TMS) was applied through a figure-of-eight coil at 20-30 sites spaced 1 cm apart over the hand area of the left motor cortex. The cortical maps of the representation of the ECR muscle constructed in this way did not change between the two motor tasks. Nevertheless, for all investigated cortical sites TMS evoked a smaller motor evoked potential (MEP) in the ECR muscles during hand clenching than during wrist extension. A similar decrease in the short-latency peak in the poststimulus time histogram (PSTH) of single ECR motor units to TMS during hand clenching was found in seven subjects (number of motor units = 35). In contrast, short-latency peaks in the PSTH evoked by electrical stimulation of the motor cortex had a similar size during the two tasks (number of motor units = 9; two subjects). Already the initial 0.5-1.0 ms of the short-latency peak evoked by TMS was depressed during hand clenching, which suggests that decreased excitability of corticospinal cells with monosynaptic projections onto ECR motor units was involved. This decreased excitability was not explained by increased intracortical inhibition, which was found to be of a similar size during hand clenching and wrist extension. The task-related changes in the efficiency of the motor cortex output are discussed in relation to the function of the wrist antagonist muscles in handling and gripping tasks.     

Modulation of H reflexes in the forearm during voluntary teeth clenching in humans

We investigated whether there is any modulation of the H reflex in the forearm during teeth clenching and how any correlation that may be found is modulated. The H reflexes of the flexor carpi radialis (FCR) and the extensor carpi radialis (ECR) muscles were evoked on the right side in five healthy adult volunteers. The H reflexes of the FCR and ECR muscles were facilitated in association with voluntary teeth clenching in a force-dependent manner (r=0.46–0.663, P<0.05). The increase in amplitude of the H reflex of the FCR muscle associated with teeth clenching started before the onset of the EMG activity of the masseter muscle. The results of the present study demonstrate that oral motor activity exerts strong influences on the motor activity of the forearm.A part of this study was presented at the World Congress on Sports Dentistry and Dental Traumatology, 22 June 2001, Boston, Massachusetts, USA     

Common drive in motor units of a synergistic muscle pair

The interaction among the motor units of the extensor carpi radialis longus (ECRL) and the extensor carpi ulnaris (ECU) muscles in man was studied during wrist extensions in which the two muscles acted as synergists. Intramuscular recordings were obtained using special quadrifilar needle electrodes. Isometric wrist extensions at 20-30% of the maximal effort were studied. The electromyographic (EMG) signals were decomposed into the individual motor-unit action potential trains comprising the signal. The interaction among motor units were characterized by the estimated time-varying mean firing rate and the cross-correlation between the time-varying mean firing rates of pairs of motor units. Pairs of motor units within each muscle as well as pairs of motor units across the muscles were considered. In-phase common fluctuations, termed common drive, were observed in the mean firing rates of motor units within each muscle, consistent with earlier work on other muscles. Common fluctuations were also observed between the firing rates of ECU and ECRL motor units albeit with a variable phase shift. The existence of common drive across synergistic muscles was interpreted as implying that the CNS considers the muscles as a functional unit when they act as synergists.     

Neural compensation for compliant loads during rhythmic movement

An experiment was performed to characterise the movement kinematics and the electromyogram (EMG) during rhythmic voluntary flexion and extension of the wrist against different compliant (elastic-viscous-inertial) loads. Three levels of each type of load, and an unloaded condition, were employed. The movements were paced at a frequency of 1 Hz by an auditory metronome, and visual feedback of wrist displacement in relation to a target amplitude of 100 degree was provided. Electromyographic recordings were obtained from flexor carpi radialis (FCR) and extensor carpi radialis brevis (ECR). The movement profiles generated in the ten experimental conditions were indistinguishable, indicating that the CNS was able to compensate completely for the imposed changes in the task dynamics. When the level of viscous load was elevated, this compensation took the form of an increase in the rate of initial rise of the flexor and the extensor EMG burst. In response to increases in inertial load, the flexor and extensor EMG bursts commenced and terminated earlier in the movement cycle, and tended to be of greater duration. When the movements were performed in opposition to an elastic load, both the onset and offset of EMG activity occurred later than in the unloaded condition. There was also a net reduction in extensor burst duration with increases in elastic load, and an increase in the rate of initial rise of the extensor burst. Less pronounced alterations in the rate of initial rise of the flexor EMG burst were also observed. In all instances, increases in the magnitude of the external load led to elevations in the overall level of muscle activation. These data reveal that the elements of the central command that are modified in response to the imposition of a compliant load are contingent, not only upon the magnitude, but also upon the character of the load.     

Task dependent gain regulation of spinal circuits projecting to the human flexor carpi radialis

In humans, the flexor carpi radialis (FCR) and extensor carpi radialis (ECR) muscles act as antagonists during wrist flexion-extension and as functional synergists during radial deviation. In contrast to the situation in most antagonist muscle pairs, Renshaw cells innervated by the motor neurons of each muscle inhibit the motoneurons, but not Ia inhibitory interneurons, of the opposite motor pool. Here we compared gain regulation of spinal circuits projecting to FCR motoneurons during two tasks: flexion and radial deviation of the wrist. We also investigated the functional consequences of this organisation for maximal voluntary contractions (MVCs). Electromyographic (EMG) recordings were taken from FCR, ECR longus and ECR brevis using fine-wire electrodes and electrical stimulation was delivered to the median and radial nerves. Ten volunteers participated in three experiments. 1. To study the regulation of the Renshaw cell-mediated, inhibitory pathway from ECR to FCR motoneurons, forty stimuli were delivered to the radial nerve at 50% of the maximal M-wave amplitude for ECR brevis. Stimuli were delivered during both isometric wrist flexions and radial deviation actions with an equivalent EMG amplitude in FCR (approximately 5% wrist flexion MVC). 2. To explore the homonymous Ia afferent pathway to FCR motoneurons, 50 stimuli were delivered to the median nerve at intensities ranging from below motor threshold to at least two times that which evoked a maximal M-wave during wrist flexion and radial deviation (matched FCR EMG at approximately 5% wrist flexion MVC). 3. EMG amplitude was measured during MVCs in wrist flexion, extension and radial deviation.There was no significant difference in the inhibition of FCR EMG induced via ECR-coupled Renshaw cells between radial deviation and wrist flexion. However, the mean FCR H-reflex amplitude was significantly (P<0.05) greater during wrist flexion than radial deviation. Furthermore, EMG amplitude in FCR and ECR brevis was significantly (P<0.05) greater during MVCs in wrist flexion and extension (respectively) than radial deviation. ECR longus EMG was significantly greater during MVCs in radial deviation than extension. These results indicate that the gain of the Renshaw-mediated inhibitory pathway between ECR and FCR motoneurons is similar for weak flexion and radial deviation actions. However, the gain of the H-reflex pathway to FCR is greater during wrist flexion than radial deviation. Transmission through both of these pathways probably contributes to the inability of individuals to maximally activate FCR during radial deviation MVCs.     

Relationship between the velocity of illusory hand movement and strength of MEG signals in human primary motor cortex and left angular gyrus

We studied the relationship between the velocity of movement illusion and the activity level of primary motor area (M1) and of the left angular gyrus (AG) in humans. To induce illusory movement perception, we applied co-vibration at different frequencies on tendons of antagonistic muscle groups. Since it is well established that the velocity of illusory movement is related to the difference in vibration frequency applied to two antagonistic muscles, we compared magnetoencephalography (MEG) signals recorded in two conditions of co-vibration: in the “fast illusion” condition a frequency difference of 80 Hz was applied on the tendons of the right wrist extensor and flexor muscle groups, whereas in the “slow illusion” condition a frequency difference of 40 Hz was applied on the same muscle groups. The dipole strength, reflecting the activity level of structures, was measured over M1 and the left AG in two different time-periods: 0–400 and 400–800 ms in each condition. Our results showed that the activity level of the AG was similar in both conditions whatever the time-period, whereas the activity level of M1 was higher in the “fast illusion” condition compared to the “slow illusion” condition from 400 ms after the vibration onset only. The data suggest that the two structures differently contributed to the perception of illusory movements. Our hypothesis is that M1 would be involved in the coding of cinematic parameters of the illusory movement but not the AG.     

The influence of experimental muscle pain on motor unit activity during low-level contraction

In the present study we compared motor unit (MU) activity in a painful extensor carpi ulnaris (ECU) muscle to that of a pain-free control. According to the pain adaptation model the activity of the painful ECU muscle may be inhibited and its antagonist activity increased during wrist extension performed as a pre-defined low-force ramp. The pre-defined low force may then be maintained by increased activity in the pain-free synergist muscles such as the extensor carpi radialis (ECR) muscle. Nine females (31–47 years old) participated in the study. Maximal voluntary contraction (MVC) of the wrist extensors was performed. A catheter was inserted into the ECU muscle to allow the injection of hypertonic saline to evoke muscle pain, and a concentric needle was inserted for the recording of MU activity. Surface electromyograms were recorded from a synergist and an antagonist (ECR and flexor carpi radialis) to the painful ECU muscle. A force ramp of isometric wrist extensions up to 10% MVC, with a force increase of 1% MVC · s⁻¹, were performed followed by 60 s of sustained contraction at 10% MVC. The number of MUs recruited was almost identical for baseline and with pain, and no effect of experimental muscle pain was found on the properties of the MUs (amplitude, area) or their firing characteristics (mean firing rate, firing variability) during low-force ramp contraction. During the sustained 10% MVC, no effect of pain was found for concentric or surface EMG of the forearm muscles. At low force levels no pain-induced modulations were found in MU activity, when the mechanical condition was similar to that of a control situation. Accepted: 30 May 2000     

人腕关节动力肌腱的力臂值及意义探讨

目的：测量人腕关节主要动力肌腱在腕运动过程中的力臂的大小。方法：运用７只新鲜成人尸体上肢标本,将运动腕关节的肌腱和旋转电压计相连,在腕关节分别从屈曲至伸直,桡偏至尺偏过程中,用计算机同时记录腕动力肌腱滑动距离和腕运动角度,根据腱滑动距离和腕运动角度计算出肌腱平均力臂和力臂变化。结果：桡侧腕长伸肌腱的腕桡偏力臂最大,桡侧腕短伸肌腱的伸腕力臂最大,尺侧腕伸肌腱和腕尺偏力臂最大,而伸腕力臂很小,桡侧、尺     

The effects of kinesthetic illusory sensation induced by a visual stimulus on the corticomotor excitability of the leg muscles

A novel method of visual stimulus, reported by Kaneko et al. [14], induced a vivid kinesthetic illusion and increased the corticomotor excitability of the finger muscles without any overt movement. To explore the effect of this method on the lower limbs, motor evoked potentials (MEP) were recorded from the left tibialis anterior (TA) and soleus muscles using transcranial magnetic stimulation (TMS). A computer screen that showed the moving image of an ankle movement was placed over the subject's leg, and its position was modulated to induce an illusory sensation that the subject's own ankle was moving (illusion condition). TMS was delivered at rest and at two different times during the illusion condition (ankle dorsiflexion phase: illusion-DF; ankle plantarflexion phase: illusion-PF). The MEP amplitude of the TA, which is the agonist muscle for ankle dorsiflexion, was significantly increased during the illusion-DF condition. This indicated that the visual stimulus showing the moving image of an ankle movement could induce a kinesthetic illusion and selectively increase the corticomotor excitability in an agonist muscle for an illusion, as was previously reported for an upper limb. The MEP amplitude of the soleus, which is the agonist muscle for ankle plantarflexion, increased during the illusion-PF condition, but not significantly. Because of the vividness of the illusory sensation was significantly greater during the illusion-DF condition than the illusion-PF condition, we concluded that the vividness of the illusory sensation had a crucial role in increasing corticomotor excitability.     

Perceptual and motor effects of agonist-antagonist muscle vibration in man

Summary Perceptual and motor effects of vibration applied simultaneously to the distal tendons of the Biceps and Triceps muscles, in isometric conditions and without sight of the stimulated arm, have been studied in human volunteers. Motor effects, measured by surface EMG, are inexistent when the flexor and extensor muscles are simultaneously vibrated at the same frequency. However, EMG activity appears in the muscle being vibrated at the lower frequency when simultaneous vibration is applied at different frequencies. The sensations felt by the subjects were reproduced by the nonvibrated arm and recorded by a goniometer. The studies show that the velocity and the amplitude of the ilusory movement is related to the difference in vibration frequency applied to the two muscles. The direction of movement felt (flexion or extension) is that produced by shortening of the muscle being vibrated at the lower frequency. When the two vibration frequencies are the same, there is either no sensation of movement, or a sensation of very slow movement. These results support the notion that the sensation of movement at a joint may be derived from a central processing of the proprioceptive inflow data obtained from flexor and extensor muscles. This interpretation may also be valid for the results obtained earlier by vibration of a single muscle. Furthermore, it is coherent with data on spindle afferent fibres obtained by microneurography in man during passive or active movements.This work was supported by grants from the Ministère de l'Industrie et de la Recherche     

Further insight into the task-dependent excitability of motor evoked potentials in first dorsal interosseous muscle in humans

We have reexamined the contradictory evidence in which task-dependent excitation of motor evoked potentials (MEPs) in the first dorsal interosseous (FDI) muscle was stronger with increasingly more complex finger tasks than with individual finger movement tasks. In the first step of the experiment, based on previous findings, we investigated remarkable functional differences between intrinsic and extrinsic hand muscles during complex finger tasks (precision and power grip). During the performance of the tasks, the optimal stimulus intensity of the transcranial magnetic stimulation (TMS) was applied to the contralateral motor cortex. MEPs of the FDI, extensor carpi radialis (ECR), and flexor carpi radialis (FCR) muscles were recorded simultaneously with increased background EMG activity step by step in both tasks. The intensity threshold of TMS was lower in the precision grip. Furthermore, the MEP amplitudes of FDI muscle dependent on the background EMG activity were different between these two tasks, i.e., MEP amplitudes and regression coefficients in a precision grip were larger than those in a power grip. Although our results for MEP amplitude and threshold in the FDI muscle were similar to previous reported evidence, the different contributions of a synergistic muscle (in particular, the ECR muscle) during performance in these tasks was new evidence. Since there were no differences in cutaneous afferent effects on both tasks, corticomotoneuronal (CM) cells connected to FDI motoneurons seemed generally to be more active during precision than power gripping, and there were different contributions from synergistic muscles during the performance of these tasks. In the second part of the experiment, the results obtained from the complex tasks were compared with those from a simple task (isolated index finger flexion). MEP amplitudes, dependent on the background EMG activity during isolated index finger flexion, varied among subjects, i.e., the relationship between the MEP amplitude and the background EMG of the FDI muscle showed individual, strategy-dependent modulation. There were several kinds of individual motor strategies for performing the isolated finger movement. The present results may explain the previous contradictory evidence related to the contribution of the CM system during coordinated finger movement.     

Primary motor cortex excitability is modulated with bimanual training

Bimanual visuomotor movement has been shown to enhance cortical motor activity in both hemispheres, especially when movements require simultaneous activation of homologous muscle groups (in-phase movement). It is currently unclear if these adaptations are specific to motor preparatory areas or if they also involve changes in primary motor cortex (M1). The present study investigated the representation of wrist muscles within motor cortex before and following bimanual movement training that was in-phase, anti-phase with or without motor preparation. Motor evoked potentials (MEPs) for the extensor carpi radialis muscle (ECR) cortical territory were acquired and analyzed before and following bimanual movement. The cortical representation was quantified and compared in terms of spatial extent and MEP amplitude, in two different experiments involving distinct movement training types. In Experiment 1, participants performed bimanual wrist flexion/extension movements to targets which involved in-phase movements, either following a 2s preparation period (In-phase preparation), or without the preparation period (In-phase no preparation). In Experiment 2, training involved antagonist muscle groups activated simultaneously (Anti-phase) with the addition of the 2s preparation period. In-phase bimanual movement enhanced the spatial representation of ECR in M1, and did not show a difference in MEP amplitude of the cortical area. It may be that simultaneous activation of homologous M1 representations in both hemispheres, in combination with activity from premotor areas, leads to a greater increase in plasticity in terms of increased M1 spatial extent of trained muscles.     

Medium-latency reflex response elicited from the flexor carpi radialis by radial nerve stimulation

The H reflex obtained from the flexor carpi radialis muscle by median nerve stimulation is a well-known monosynaptic reflex. However, the origin of the late responses is still contentious. Radial nerve stimulation was performed through the spiral groove, and EMG recording was obtained from the flexor carpi radialis (FCR) and extensor carpi radialis (ECR) muscles. An M response followed by an F response was achieved from the ECR by radial nerve stimulation; the antagonistic FCR muscle elicited a late response. A total of 25 cases were included in this study. In 22 of these cases, a response with a latency of 40.97 ± 5.35 ms was obtained from the FCR by radial nerve stimulation. When extension of the hand was restricted, the response disappeared in five of nine cases. Application of cold markedly suppressed the response and prolonged the latency of the FCR medium-latency reflex response (FCR-MLR). Oral tizanidine considerably suppressed the FCR-MLR response. Two out of eight cases did not exhibit any response. No response could be recorded from a patient with complete amputation of the right hand. The FCR-MLR is the reflex caused by stretching of the FCR muscle from radial nerve stimulation, and it is greatly influenced by group II afferents.     

Synaptic connections from large afferents of wrist flexor and extensor muscles to synergistic motoneurones in man

 Short-latency excitatory Ia reflex connections were determined between pairs of human wrist flexor and extensor muscles. Spindle Ia afferents were stimulated by either tendon tap or electrical stimulation. The activity of voluntarily activated single motor units was recorded intramuscularly from pairs of wrist flexor or extensor muscles. Cross-correlation between stimuli and the discharge of the motor units provided a measure of the homonymous or heteronymous excitatory input to a motoneurone. Homonymous motoneurone facilitation was generally stronger than that of the heteronymous motoneurones. The principal wrist flexors, flexor carpi radialis (FCR) and flexor carpi ulnaris (FCU), were tightly connected through a bidirectional short-latency reflex pathway. In contrast, the extensor carpi ulnaris (ECU) and the extensor carpi radialis (ECR) did not have similar connections. ECU motoneurones received no short-latency excitatory Ia input from the ECR. ECR motoneurones did receive excitatory Ia input from ECU Ia afferents; however, its latency was delayed by several milliseconds compared with other heteronymous Ia excitatory effects observed. The wrist and finger extensors were linked through heteronymous Ia excitatory reflexes. The reflex connections observed in humans are largely similar to those observed in the cat, with the exception of heteronymous effects from the ECU to the ECR and from the extensor digitorum communis (EDC) to the ECU, which are present only in humans. The differences in the reflex organization of the wrist flexors versus the extensors probably reflects the importance of grasping. Received: 19 August 1996 / Accepted: 6 March 1997