Suppression of soleus H-reflex amplitude is graded with frequency of rhythmic arm cycling

In humans, rhythmic arm cycling has been shown to significantly suppress the soleus H-reflex amplitude in stationary legs. The specific nature of the relationship between frequency of arm cycling and H-reflex modulation in the legs has not been explored. We speculated that the effect of arm cycling on reflexes in leg muscles is related to the neural control of arm movement; therefore, we hypothesized that a graded increase in arm cycling frequency would produce a graded suppression of the soleus H-reflex amplitude. We also hypothesized that a threshold frequency of arm cycling would be identified at which the H-reflex amplitude significantly differed from static control trials (i.e., the arms were stationary). Soleus H-reflexes were evoked in stationary legs with tibial nerve stimulation during both control and rhythmic arm cycling (0.03–2.0 Hz) trials. The results show a significant inverse linear relation between arm cycling frequency and soleus H-reflex amplitude (P < 0.05). Soleus H-reflex amplitude significantly differed from control at an average threshold cycling frequency of 0.8 Hz. The results demonstrate that increased frequency of upper limb movement increases the intensity of interlimb influences on the neural activity in stationary legs. Further, a minimum threshold frequency of arm cycling is required to produce a significant effect. This suggests that achieving a threshold frequency of rhythmic arm movement may be important to incorporate in rehabilitation strategies to engage the appropriate interlimb neural pathways.     

Phase-dependent modulation of soleus H-reflex amplitude induced by rhythmic arm cycling

Rhythmic arm cycling is known to suppress the Hoffmann (H-) reflex amplitudes in the soleus (Sol) muscles of stationary legs. However, it has remained unclear if this suppression is modulated according to the phase of movement in the cycle path or is rather a general setting of excitability level related to rhythmic movement. In the present study we investigated the phase-dependent modulation of the Sol H-reflex induced by rhythmic arm cycling by examining reflex amplitudes at 12 phases of the arm cycle movement. Arm cycling tasks consisted of bilateral, ipsilateral and contralateral movement. Additionally, data were also sampled at 12 static arm positions mimicking those occurring during movement. H-reflexes were evoked and recorded at constant motor wave amplitudes across all conditions. Suppression of Sol H-reflex amplitude was dependent upon the phase of movement (main effect p < 0.0001) during arm cycling, but not during static positioning. Results suggest that locomotor central pattern generators may contribute to the phasic reflex modulation observed in this study. The phasic modulation was more pronounced during bilateral movement, however aspects of the neural control driving this modulation were also present during ipsilateral and contralateral movement.     

Rhythmic arm cycling differentially modulates stretch and H-reflex amplitudes in soleus muscle

During rhythmic arm cycling, soleus H-reflex amplitudes are reduced by modulation of group Ia presynaptic inhibition. This suppression of reflex amplitude is graded to the frequency of arm cycling with a threshold of 0.8 Hz. Despite the data on modulation of the soleus H-reflex amplitude induced by rhythmic arm cycling, comparatively little is known about the modulation of stretch reflexes due to remote limb movement. Therefore, the present study was intended to explore the effect of arm cycling on stretch and H-reflex amplitudes in the soleus muscle. In so doing, additional information on the mechanism of action during rhythmic arm cycling would be revealed. Although both reflexes share the same afferent pathway, we hypothesized that stretch reflex amplitudes would be less suppressed by arm cycling because they are less inhibited by presynaptic inhibition. Failure to reject this hypothesis would add additional strength to the argument that Ia presynaptic inhibition is the mechanism modulating soleus H-reflex amplitude during rhythmic arm cycling. Participants were seated in a customized chair with feet strapped to footplates. Three motor tasks were performed: static control trials and arm cycling at 1 and 2 Hz. Soleus H-reflexes were evoked using single 1 ms pulses of electrical stimulation delivered to the tibial nerve at the popliteal fossa. A constant M-wave and ~6% MVC activation of soleus were maintained across conditions. Stretch reflexes were evoked using a single sinusoidal pulse at 100 Hz given by a vibratory shaker placed over the triceps surae tendon and controlled by a custom-written LabView program. Results demonstrated that rhythmic arm cycling that was effective for conditioning soleus H-reflexes did not show a suppressive effect on the amplitude of the soleus stretch reflex. We suggest this indicates that stretch reflexes are less sensitive to conditioning by rhythmic arm movement, as compared to H-reflexes, due to the relative insensitivity to Ia presynaptic inhibition.     

Effect of rhythmic arm movement on reflexes in the legs: modulation of soleus H-reflexes and somatosensory conditioning

During locomotor tasks such as walking, running, and swimming, the arms move rhythmically with the legs. It has been suggested that connections between the cervical and lumbosacral spinal cord may mediate some of this interlimb coordination. However, it is unclear how these interlimb pathways modulate reflex excitability during movement. We hypothesized that rhythmic arm movement would alter the gain of reflex pathways in the stationary leg. Soleus H-reflexes recorded during arm cycling were compared with those recorded at similar positions with the arms stationary. Nerve stimulation was delivered with the right arm at approximately 70 degrees shoulder flexion or 10 degrees shoulder extension. H-reflexes were evoked alone (unconditioned) or with sural or common peroneal nerve (CP) conditioning to decrease or increase soleus IA presynaptic inhibition, respectively. Both conditioning stimuli were also delivered with no H-reflex stimulation. H-reflex amplitudes were compared at similar M-wave amplitudes and activation levels of the soleus. Arm cycling significantly reduced (P < 0.05) unconditioned soleus H-reflexes at shoulder flexion by 21.7% and at shoulder extension by 8.8% compared with static controls. The results demonstrate a task-dependent modulation of soleus H-reflexes between arm cycling and stationary trials. Sural nerve stimulation facilitated H-reflexes at shoulder extension but not at shoulder flexion during static and cycling trials. CP nerve stimulation significantly reduced H-reflex amplitude in all conditions. Reflexes in soleus when sural and CP nerve stimulation were delivered alone, were not different between cycling and static trials; thus the task-dependent change in H reflex amplitude was not due to changes in motoneuron excitability. Therefore modulation occurred at a pre-motoneuronal level, probably by presynaptic inhibition of the IA afferent volley. Results indicate that neural networks coupling the cervical and lumbosacral spinal cord in humans are activated during rhythmic arm movement. It is proposed that activation of these networks may assist in reflex linkages between the arms and legs during locomotor tasks.     

Facilitation of soleus H-reflex amplitude evoked by cutaneous nerve stimulation at the wrist is not suppressed by rhythmic arm movement

Neural connections between the cervical and lumbosacral spinal cord may assist in arm and leg coordination during locomotion. Currently the extent to which arm activity can modulate reflex excitability of leg muscles is not fully understood. We showed recently that rhythmic arm movement significantly suppresses soleus H-reflex amplitude probably via modification of presynaptic inhibition of the IA afferent pathway. Further, during walking reflexes evoked in leg muscles by stimulation of a cutaneous nerve at the wrist (superficial radial nerve; SR) are phase and task dependent. However, during walking both the arms and legs are rhythmically active thus it is difficult to identify the locus of such modulation. Here we examined the influence of SR nerve stimulation on transmission through the soleus H-reflex pathway in the leg during static contractions and during rhythmic arm movements. Nerve stimulation was delivered with the right shoulder in flexion or extension. H-reflexes were evoked alone (unconditioned) or with cutaneous conditioning via stimulation of the SR nerve (also delivered alone without H-reflex in separate trials). SR nerve stimulation significantly facilitated H-reflex amplitude during static contractions with the arm extended and countered the suppression of reflex amplitude induced by arm cycling. The results demonstrate that cutaneous feedback from the hand on to the soleus H-reflex pathway in the legs is not suppressed during rhythmic arm movement. This contrasts with the observation that rhythmic arm movement suppresses facilitation of soleus H-reflex when cutaneous nerves innervating the leg are stimulated. In conjunction with other data taken during walking, this suggests that the modulation of transmission through pathways from the SR nerve to the lumbosacral spinal cord is partly determined by rhythmic activity of both the arms and legs.     

Neural control of rhythmic human arm movement: phase dependence and task modulation of hoffmann reflexes in forearm muscles

Although we move our arms rhythmically during walking, running, and swimming, we know little about the neural control of such movements. Our working hypothesis is that neural mechanisms controlling rhythmic movements are similar in the human lumbar and cervical spinal cord. Thus reflex modulation during rhythmic arm movement should be similar to that seen during leg movement. Our main experimental hypotheses were that the amplitude of H-reflexes in the forearm muscles would be modulated during arm movement (i.e., phase-dependent) and would be inhibited during cycling compared with static contraction (i.e., task-dependent). Furthermore, to determine the locus of any modulation, we tested the effect that active and passive movement of the ipsilateral (relative to stimulated arm) and contralateral arm had on H-reflex amplitude. Subjects performed rhythmic arm cycling on a custom-made hydraulic ergometer in which the two arms could be constrained to move together (180 degrees out of phase) or could rotate independently. Position of the stimulated limb in the movement cycle is described with respect to the clock face. H-reflexes were evoked at 12, 3, 6, and 9 o'clock positions during static contraction as well as during rhythmic arm movements. Reflex amplitudes were compared between tasks at equal M wave amplitudes and similar levels of electromyographic (EMG) activity in the target muscle. Surface EMG recordings were obtained bilaterally from flexor carpi radialis as well as from other muscles controlling the wrist, elbow, and shoulder. Compared with reflexes evoked during static contractions, movement of the stimulated limb attenuated H-reflexes by 50.8% (P < 0.005), 65.3% (P < 0.001), and 52.6% (P < 0.001) for bilateral, active ipsilateral, and passive ipsilateral movements, respectively. In contrast, movement of the contralateral limb did not significantly alter H-reflex amplitude. H-reflexes were also modulated by limb position (P < 0.005). Thus task- and phase-dependent modulation were observed in the arm as previously demonstrated in the leg. The data support the hypothesis that neural mechanisms regulating reflex pathways in the moving limb are similar in the human upper and lower limbs. However, the inhibition of H-reflex amplitude induced by contralateral leg movement is absent in the arms. This may reflect the greater extent to which the arms can be used independently.     

Enhancement of arm and leg locomotor coupling with augmented cutaneous feedback from the hand

Cutaneous feedback from the hand could assist with coordination between the arms and legs during locomotion. Previously we used a reduced walking model of combined arm and leg (ARM&LEG) cycling to examine the separate effects of rhythmic arm (ARM) and leg (LEG) movement. Here we use this same paradigm to test the modulation H-reflexes with and without interlimb cutaneous conditioning evoked by stimulating a nerve innervating the hand (superficial radial, SR). It was hypothesized that both ARM and LEG would contribute significantly to suppression of H-reflex amplitude during ARM&LEG. We also predicted a conservation of interlimb cutaneous conditioning during movement and an interaction between arm and leg rhythmic movement control. Subjects were seated in a recumbent ARM&LEG cycle ergometer and maintained a low-level soleus contraction for all tasks. H-reflex amplitude was facilitated by cutaneous conditioning evoked by stimulation of the SR nerve. H-reflex amplitudes were taken from recruitment curves and included modulation of 50% H max and H max. The suppressive effect of arm was less than that for LEG and ARM&LEG, while suppression during LEG and ARM&LEG were generally equivalent. For H-reflexes conditioned by cutaneous input, amplitudes during ARM&LEG instead were in between those for ARM and LEG modulation. Multiple regression analysis revealed a significant contribution for arm only in trials when SR stimulation was used to condition H-reflex amplitudes. We suggest that there is a measurable interaction between neural activity regulating arm and leg movement during locomotion that is specifically enhanced when cutaneous input from the hand is present.     

Short-term plasticity of spinal reflex excitability induced by rhythmic arm movement

Rhythmic arm movement reduces Hoffmann (H)-reflex amplitudes in leg muscles by modulation of presynaptic inhibition in group Ia transmission. To date only the acute effect occurring during arm movement has been studied. We hypothesized that the excitability of soleus H-reflexes would remain suppressed beyond a period of arm cycling conditioning. Subjects used a customized arm ergometer to perform rhythmic 1-Hz arm cycling for 30 min. H-reflexes were evoked before, during, and after arm cycling via stimulation of the tibial nerve in the popliteal fossa. The most important finding was that the H-reflex amplitudes were significantly suppressed during and 相似文献   

Neural coupling between the upper and lower limbs in humans

The aim of this study was to establish the effects of active sinusoidal ipsilateral and contralateral upper limb flexion, extension, abduction, and adduction with elbows extended on the right soleus H-reflex with subjects seated and standing. Reflex effects were also established when both arms moved synchronously in a reciprocal pattern with elbows flexed in seated and standing subjects. Sinusoidal arm movements were timed to a metronome and performed at 0.2 Hz. Soleus H-reflexes were elicited only once (every 4s) in every movement cycle of the upper limbs. Position of arms, and activity of shoulder muscles were recorded through twin-axis goniometers and surface electromyography (EMG), respectively. We found that in seated subjects, regardless the direction of the active movement or the upper limb being moved, the soleus H-reflex was depressed. In standing subjects, a reflex depression was observed during extension, abduction, and adduction of the ipsilateral and contralateral upper limbs. Muscles were active during arm flexion and abduction in all directions of arm movement with subjects either seated or standing. It is suggested that arm movement might be incorporated in the rehabilitation training of people with a supraspinal or spinal cord lesion, since it can benefit motor recovery by decreasing spinal reflex excitability of the legs in these patients.     

Rhythmic arm cycling modulates Hoffmann reflex excitability differentially in the ankle flexor and extensor muscles

Rhythmic arm movement significantly suppresses H-reflex amplitude in the soleus (SOL) muscle. This is evidence of neural linkages between the arms and legs which can be exploited during locomotion and have been ascribed to the descending effects of CPGs for arm cycling. However, the generalizability of the effects of arm movement on reflex excitability within the lower leg musculature has not been confirmed, as findings have been limited solely to the ankle extensor group. Here we tested the hypothesis that rhythmic arm movement similarly modulates H-reflex amplitude in both the ankle flexors and extensors by observing responses in the SOL and tibialis anterior (TA) muscles. SOL and TA H-reflex recruitment curves were recorded bilaterally during control and 1 Hz arm cycling conditions. Our results showed significant suppression in H-reflex amplitude (H_max) in the SOL muscle in both the dominant and non-dominant legs during arm movement. However, results also revealed an unpredicted bidirectional (i.e. either suppression or facilitation) modulation of TA reflex amplitude that was not present in the SOL muscle. These findings suggest a differential regulation of ankle flexor and extensor H-reflex responses during rhythmic arm movement. This may be the result of differences in CPG output to the flexors and extensors during rhythmic movement, as well as increased involvement of cortical drive to the flexors relative to the extensors during rhythmic movement. These findings may be pertinent to future investigation of rehabilitative therapies that involve facilitative modulation of ankle flexor motor responses.     

Rhythmic leg cycling modulates forearm muscle H-reflex amplitude and corticospinal tract excitability

Rhythmic arm cycling leads to suppression of H-reflexes in both leg and arm muscles, and a reduction in the excitability of corticospinal projections to the forearm flexors. It is unknown, however, whether leg cycling modulates excitability in neural projections to the arms. Here we studied the extent to which rhythmic movement of the legs alters reflex (Experiment 1) and corticospinal (Experiment 2) transmission to arm muscles. In experiment 1, flexor carpi radialis (FCR) H-reflex recruitment curves were recorded with the legs static, and during rhythmic leg movement, while the FCR was both contracted and relaxed. The results indicate that rhythmic leg movement suppresses reflex transmission, both when FCR is at rest and during tonic contraction, but that the effect is not phase-dependent. In experiment 2, we used transcranial magnetic stimulation (TMS) to elicit motor-evoked potentials in the contracted and relaxed FCR during static leg, and leg cycling conditions. Sub-threshold TMS was also used to condition H-reflexes in order to provide specific information about cortical excitability during leg cycling. Both resting and tonically contracting arm muscles showed a greater corticospinal excitability during leg cycling than during the static leg condition. The magnitude of TMS facilitation of H-reflexes was similar during leg cycling and rest, suggesting a considerable sub-cortical component to the increased corticospinal excitability. The results suggest a differential regulation of afferent and descending projections to the arms during leg cycling, and are consistent with the idea that there is a loose, but significant, neural coupling between the arms and legs during rhythmic movement.     

Low frequency depression of H-reflexes in humans with acute and chronic spinal-cord injury

We measured low-frequency depression of soleus H-reflexes in individuals with acute (n=5) and chronic (n=7) spinal-cord injury and in able-bodied controls (n=7). In one acute subject, we monitored longitudinal changes in low-frequency depression of H-reflexes over 44 weeks and examined the relationship between H-reflex depression and soleus-muscle fatigue properties. Soleus H-reflexes were elicited at 0.1, 0.2, 1, 5, and 10 Hz. The mean peak-to-peak amplitude of ten reflexes at each frequency was calculated, and values obtained at each frequency were normalized to 0.1 Hz. H-reflex amplitude decreased with increasing stimulation frequency in all three groups, but H-reflex suppression was significantly larger in the able-bodied and acute groups than in the chronic group. The acute subject who was monitored longitudinally displayed reduced low-frequency depression with increasing time post injury. At 44 weeks post injury, the acute subject's H-reflex depression was similar to that of chronic subjects, and his soleus fatigue index (assessed with a modified Burke fatigue protocol) dropped substantially, consistent with transformation to faster muscle. There was a significant inverse correlation over the 44 weeks between the fatigue index and the mean normalized H-reflex amplitude at 1, 5, and 10 Hz. We conclude that: (1) the chronically paralyzed soleus muscle displays impaired low-frequency depression of H-reflexes, (2) attenuation of rate-sensitive depression in humans with spinal-cord injury occurs gradually, and (3) changes in H-reflex excitability are generally correlated with adaptation of the neuromuscular system. Possible mechanisms underlying changes in low-frequency depression and their association with neuromuscular adaptation are discussed.     

Corticospinal excitability is lower during rhythmic arm movement than during tonic contraction

Humans perform rhythmic, locomotor movements with the arms and legs every day. Studies using reflexes to probe the functional role of the CNS suggest that spinal circuits are an important part of the neural control system for rhythmic arm cycling and walking. Here, by studying motor-evoked potentials (MEPs) in response to transcranial magnetic stimulation (TMS) of the motor cortex, and H-reflexes induced by electrical stimulation of peripheral nerves, we show a reduction in corticospinal excitability during rhythmic arm movement compared with tonic, voluntary contraction. Responses were compared between arm cycling and tonic contraction at four positions, while participants generated similar levels of muscle activity. Both H-reflexes and MEPs were significantly smaller during arm cycling than during tonic contraction at the midpoint of arm flexion (F = 13.51, P = 0.006; F = 11.83, P = 0.009). Subthreshold TMS significantly facilitated the FCR H-reflex during tonic contractions, but did not significantly modulate H-reflex amplitude during arm cycling. The data indicate a reduction in the responsiveness of cells constituting the fast, monosynaptic, corticospinal pathway during arm cycling and suggest that the motor cortex may contribute less to motor drive during rhythmic arm movement than during tonic, voluntary contraction. Our results are consistent with the idea that subcortical regions contribute to the control of rhythmic arm movements despite highly developed corticospinal projections to the human upper limb.     

Bilateral soleus H-reflexes in humans elicited by simultaneous trains of stimuli: symmetry, variability, and covariance

Experiments using electrical and mechanical activation of spinal reflexes have contributed important results toward the understanding of neuronal and synaptic dynamics involved in spinal neural circuits as well as their response to different inputs. In this work, data obtained from the simultaneous stimulation of both legs are analyzed to provide information on the degree of symmetry of the respective spinal reflex circuits and on the characteristics of reflex variability. H-reflexes recorded from relaxed muscles show a frequency-dependent amplitude depression when elicited by a train of stimuli. This effect has been attributed to homosynaptic depression. Soleus H-reflexes were recorded in response to trains of simultaneous stimuli applied to both legs in right-handed subjects that were sitting in a relaxed state. The first objective was to verify the existence of asymmetries in H-reflex parameters obtained from the two legs. We measured the mean, variance, and coefficient of variation of the depressed H-reflex amplitudes and the time constant of decay toward the depressed plateau. The second objective was the analysis of the time correlation of subsequent H-reflex amplitudes in a long train of responses recorded from a given leg. The statistical dependence of H-reflex amplitudes in the long trains recorded from both legs was also investigated. Data obtained from preliminary experiments showed that there was no effect of a given stimulus on the contralateral leg applied simultaneously or 1 s before, therefore validating the simultaneous stimulation paradigm. Paired t-tests indicated that several parameters measured bilaterally from soleus H-reflex trains of right-handed subjects were not statistically different in the overall, although individually there were statistically significant asymmetries, toward either the right or left leg. Sequences of H-reflex amplitudes, as measured by the auto-covariance, were either white or had a memory ranging from 2 up to 50 s. This indicates that the random fluctuations in presynaptic inhibition and/or postsynaptic inputs to motoneurons may have either fast or slow time courses. The average auto-covariance sequences of the right and left legs, computed from all subjects, were practically superposable. The cross-covariance between the bilateral H-reflex amplitudes showed a statistically significant peak at zero lag in some experiments, suggesting a correlation between the synaptic inputs to the Ia-motoneuron systems of the soleus muscles of both legs.     

Cutaneous reflexes during rhythmic arm cycling are insensitive to asymmetrical changes in crank length

The neural control of a movement depends upon the motor task performed. To further understand the neural regulation of different variations of the same type of movement, we created three dissimilar bilateral rhythmic arm cycling tasks by unilaterally manipulating crank length (CL). Modulation in the amplitude and sign of cutaneous reflexes was used as an index of neural control. Neurologically intact subjects performed three bilateral cycling trials at ∼1 Hz with the ipsilateral crank arm at one of three different lengths. Cutaneous reflexes were evoked during each trial with trains (5 × 1.0 ms pulses at 300 Hz) of electrical stimulation delivered to the superficial radial nerve at the ipsilateral wrist. EMG recordings were made bilaterally from muscles acting at the shoulder, elbow, and wrist. Analysis was conducted after phase-averaging contingent upon the timing of stimulation in the movement cycle. CL variation created an asymmetrical cycling pattern and produced significant changes in the range of motion at the ipsilateral shoulder and elbow. Background EMG amplitude in muscles of the contralateral arm generally increased significantly as CL decreased. Therefore at a given phase in the movement cycle, the background EMG was different between the three cycling trials. In contrast, cutaneous reflex amplitudes in muscles of both arms were similar at each phase of the movement cycle between the different CLs trials at both early and middle latencies. This was particularly evident in muscles ipsilateral to nerve stimulation. We suggest that variations of arm cycling that primarily yield significant changes in the amplitude of muscle activity do not require significant task-specific change in neural control.     

Spacing practice sessions across days earlier rather than later in training improves performance of a visuomotor skill

Reduced depression of transmitter release from Ia afferents following previous activation (post-activation depression) has been suggested to be involved in the pathophysiology of spasticity. However, the effect of this mechanism on the myotatic reflex and its possible contribution to increased reflex excitability in spastic participants has not been tested. To investigate these effects, we examined post-activation depression in Soleus H-reflex responses and in mechanically evoked Soleus stretch reflex responses. Stretch reflex responses were evoked with consecutive dorsiflexion perturbations delivered at different intervals. The magnitude of the stretch reflex and ankle torque response was assessed as a function of the time between perturbations. Soleus stretch reflexes were evoked with constant velocity (175°/s) and amplitude (6°) plantar flexion perturbations. Soleus H-reflexes were evoked by electrical stimulation of the tibial nerve in the popliteal fossa. The stretch reflex and H-reflex responses of 30 spastic participants (with multiple sclerosis or spinal cord injury) were compared with those of 15 healthy participants. In the healthy participants, the magnitude of the soleus stretch reflex and H-reflex decreased as the interval between the stimulus/perturbation was decreased. Similarly, the stretch-evoked torque decreased. In the spastic participants, the post-activation depression of both reflexes and the stretch-evoked torque was significantly smaller than in healthy participants. These findings demonstrate that post-activation depression is an important factor in the evaluation of stretch reflex excitability and muscle stiffness in spasticity, and they strengthen the hypothesis that reduced post-activation depression plays a role in the pathophysiology of spasticity.     

How does action resist visual illusion? Uncorrected oculomotor information does not account for accurate pointing in peripersonal space

The amplitudes and signs of cutaneous reflexes are modulated during rhythmic movements of the arms and legs (during walking and arm or leg cycling for instance). This reflex modulation is frequently independent of the background muscle activity and may involve central pattern generator (CPG) circuits. The purpose of the present study was to investigate the nature and degree of coupling between the upper limbs during arm cycling, with regard to the regulation of cutaneous reflexes. Responses to electrical stimulations of the right, superficial radial nerve (five 1 ms pulses, 300 Hz) were recorded bilaterally in six arm muscles of eight participants during arm cycling involving only the limb ipsilateral to the stimulation, only the limb contralateral to the stimulation, and bilateral movement when the limbs were both in-phase and 180° out of phase. The pattern of cutaneous reflex modulation throughout the arm cycle was independent of the functional state of the limb contralateral to the recording site, irrespective of whether recordings were made ipsilateral or contralateral to the stimulation. Furthermore, cutaneous reflexes were significantly (p<0.05) modulated with arm position in only 8% of cases in which the limb containing the responding muscle was either stationary or being moved passively by the experimenter. The results show that there is relatively weak coupling between the arms with regard to the regulation of cutaneous reflexes during rhythmic, cyclical arm movements. This suggests a loose connection between the CPGs for each arm that regulate muscle activity and reflex amplitude during rhythmic movement.     

Corticospinal excitability during laughter: implications for cataplexy and the comparison with REM sleep atonia

Cataplexy is usually seen as rapid eye movement (REM) sleep atonia occurring at an inopportune moment. REM sleep atonia is the result of postsynaptic inhibition, i.e. inhibition of alpha motor neurones. Although this may explain the suppression of H-reflexes during REM sleep, cataplexy and laughter, it is not the only explanation. Presynaptic inhibition, in which afferent impulses are prevented from reaching motor neurones, is an alternative. Testing H-reflexes and magnetic-evoked potentials (MEPs) helps to tell them apart: in postsynaptic inhibition MEPs and H-reflexes change in tandem, while H-reflexes may decrease independent of MEPs with other inhibition modes. We studied motor inhibition during laughter, the strongest trigger for cataplexy. H-reflexes were evoked every 2 s in the soleus muscle in 10 healthy subjects watching comical video fragments. MEPs were evoked when H-reflexes decreased during laughter, and, as a control, when subjects did not laugh. Pairs of MEPs and the immediately preceding H-reflexes were studied. Compared with the control condition, laughter caused mean MEP area to increase by 60% (P=0.006) and mean H-reflex amplitude to decrease by 33% (P=0.008). This pattern proves that postsynaptic inhibition cannot have been the sole influence. The findings do not prove which mechanisms are involved; one possibility is that the decrease in H-reflex amplitude was the result of presynaptic inhibition, and that cortical and/or spinal facilitation accounted for increased MEPs. Regardless, the pattern differs fundamentally from the reported mechanism of REM sleep atonia. Existing scanty data on cataplexy suggest a pattern of H-reflexes and MEPs similar to that during laughter, but this needs further study.     

Changes in segmental and motor cortical output with contralateral muscle contractions and altered sensory inputs in humans

Motor or sensory activity in one arm can affect the other arm. We tested the hypothesis that a voluntary contraction can affect the motor pathway to the contralateral homologous muscle and investigated whether alterations in sensory input might mediate such effects. Responses to transcranial magnetic stimulation [motor-evoked potentials (MEPs)], stimulation of the descending tracts [cervicomedullary MEPs (CMEPs)], and peripheral nerve stimulation (H-reflex) were recorded from the relaxed right flexor carpi radialis (FCR), while the left arm underwent unilateral interventions (5 s duration) that included voluntary contraction, muscle contraction evoked through percutaneous stimulation, tendon vibration, and cutaneous and mixed nerve stimulation. During moderate to strong voluntary wrist flexion on the left, MEPs in the right FCR increased, CMEPs were unaffected, and the H-reflex was depressed. These results are consistent with an increase in excitability of the motor cortex, no effect on the motoneuron pool, and presynaptic inhibition of Ia afferents. In contrast, percutaneous muscle stimulation facilitated both MEPs and the H-reflex. However, muscle contraction produced by a combination of voluntary effort and electrical stimulation also reduced the contralateral H-reflex. After voluntary contractions, the H-reflex remained depressed for 35 s, but after stimulation-evoked contractions, it rapidly returned to baseline. Under both conditions, MEPs recovered rapidly. After voluntary contractions, CMEPs were also depressed for approximately 10 s despite their lack of change during contractions. Wrist tendon vibration (100 Hz) did not affect, and 20-Hz median nerve stimulation or forearm medial cutaneous nerve stimulation mildly facilitated, the H-reflex without affecting MEPs. Voluntary wrist extension, similarly to wrist flexion, increased MEPs and depressed H-reflexes. However, ankle dorsiflexion facilitated the H-reflex akin to the Jendrassik maneuver. These data suggest that a unilateral voluntary muscle contraction has contralateral effects at both cortical and segmental levels and that the segmental effects are not replicated by stimulated muscle contraction or by input from muscle spindles or non-nociceptive cutaneous afferents.     

Multi-frequency arm cycling reveals bilateral locomotor coupling to increase movement symmetry

Upright stance has allowed for substantial flexibility in how the upper limbs interact with each other: the arms can be coordinated in alternating, synchronous, or asymmetric patterns. While synchronization is thought to be the default mode of coordination during bimanual movement, there is little evidence for any bilateral coupling during locomotor-like arm cycling movements. Multi-frequency tasks have been used to reveal bilateral coupling during bimanual movements, thus here we used a multi-frequency task to determine whether the arms are coupled during arm cycling. It was hypothesized that bilateral coupling would be revealed as changes in background EMG and cutaneous reflexes when temporal coordination was altered. Twelve subjects performed arm cycling at 1 and 2 Hz with one arm while the contralateral arm was either at rest, cycling at the same frequency, or cycling at a different frequency (i.e., multi-frequency cycling with one arm at 1 Hz and the other at 2 Hz). To evoke reflexes, the superficial radial nerve was stimulated at the wrist. EMG was collected continuously from muscles of both arms. Results showed that background EMG in the lower frequency arm was amplified while reflex amplitudes were unaltered during multi-frequency cycling. We propose that neural coupling between the arms aids in equalizing muscle activity during asymmetric tasks to permit stable movement. Conversely, such interactions between the arms would likely be unnecessary in determining a reflexive response to a perturbation of one arm. Therefore, bilateral coupling was expressed when it was relevant to symmetry.