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   

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.     

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.     

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.     

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.     

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.     

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.     

Segmental reflexes and ankle joint stiffness during co-contraction of antagonistic ankle muscles in man

The size of soleus H-reflexes and short-latency stretch reflexes was measured at different levels of plantar flexion or co-contraction (simultaneous activation of dorsi- and plantar flexors) in seven healthy subjects. In four of seven subjects the short-latency stretc reflex was smaller during weak co-contraction than during isolated plantar flexion at matched background electromyogram (EMG) levels in the soleus muscle. In three of these four subjects the stretch reflex was larger during strong co-contraction than during plantar flexion, whereas it had the same size during the two tasks in the last subject. In the remaining subjects the stretch reflex either had the same size or was larger at all levels of co-contraction than at similar levels of plantar flexion. In contrast, the H-reflex was found to decrease with co-contraction at all contraction levels in all subjects. The decrease in the reflexes during weak co-contraction might be caused by presynaptic inhibition of Ia afferents. It is unclear why only the H-reflex decreased during strong co-contraction. The stiffness of the ankle joint was measured from the torque increment following the stretch of the plantar flexors divided by the stretch amplitude. In all subjects the total stiffness of the ankle joint was larger during strong co-contraction than during plantar flexion of similar strength. The stiffness was smaller during weak co-contraction than during weak plantar flexion in three out of seven subjects. The medial gastrocnemius muscle was more active at a given level of soleus activity during the co-contraction task than during the isolated plantar flexion task. It is suggested that the increase in the stiffness during co-contraction as compared to isolated plantar flexion was mainly due to the mechanical contribution of the activity in the tibialis anterior and medial gastrocnemius muscles. The decrease in stiffness during weak co-contraction was, in contrast, most likely mainly caused by modulation of reflex stiffness.     

Rhythmic arm cycling produces a non-specific signal that suppresses Soleus H-reflex amplitude in stationary legs

Rhythmic arm cycling significantly suppresses Hoffmann (H-) reflex amplitude in Soleus muscles of stationary legs. The specific parameters of arm cycling contributing to this suppression, however, are unknown. Between the arms or legs, movement results in suppression of the H-reflex that is specifically related to the phase of movement and the locus of limb movement. We speculated that the effects of arm movement features on H-reflexes in the leg would be similar and hypothesized that the Soleus H-reflex suppression evoked by arm movement would therefore be specifically related to: (1) phase of the movement; (2) the locus of the movement (i.e., ipsilateral or contralateral arm); (3) range of arm motion; and (4) frequency of arm cycling. Participants performed bilateral arm cycling at 1 and 2 Hz with short and long-crank lengths. Ipsilateral and contralateral arm cycling was also performed at 1 Hz with a long-crank length. Soleus H-reflexes were evoked at four equidistant phases and comparisons were made while maintaining similar evoked motor waves and Soleus activation. Our results show that comparable suppressive effects were seen at all phases of the arm movement: there was no phase-dependence. Further, bilateral or unilateral (whether ipsi- or contralateral arm) cycling yielded equivalent suppression of the H-reflex amplitude. Cycling at 2 Hz resulted in a significantly larger suppression than with 1 Hz cycling. We conclude that a general, rather than a specific, signal related to the command to produce rhythmic arm muscle activity mediates the suppression of Soleus H-reflex during arm cycling.     

Excitability changes in human forearm corticospinal projections and spinal reflex pathways during rhythmic voluntary movement of the opposite limb

Rhythmic movements brought about by the contraction of muscles on one side of the body give rise to phase-locked changes in the excitability of the homologous motor pathways of the opposite limb. Such crossed facilitation should favour patterns of  bimanual coordination in which homologous muscles are engaged simultaneously, and disrupt those in which the muscles are activated in an alternating fashion. In order to examine these issues, we obtained responses to transcranial magnetic stimulation (TMS), to stimulation of the cervicomedullary junction (cervicomedullary-evoked potentials, CMEPs), to peripheral nerve stimulation (H-reflexes and f-waves), and elicited stretch reflexes in the relaxed right flexor carpi radialis (FCR) muscle during rhythmic (2 Hz) flexion and extension movements of the opposite (left) wrist. The potentials evoked by TMS in right FCR were potentiated during the phases of movement in which the left FCR was most strongly engaged. In contrast, CMEPs were unaffected by the movements of the opposite limb. These results suggest that there was systematic variation of the excitability of the motor cortex ipsilateral to the moving limb. H-reflexes and stretch reflexes recorded in right FCR were modulated in phase with the activation of left FCR. As the f-waves did not vary in corresponding fashion, it appears that the phasic modulation of the H-reflex was mediated by presynaptic inhibition of Ia afferents. The observation that both H-reflexes and f-waves were depressed markedly during movements of the opposite indicates that there may also have been postsynaptic inhibition or disfacilitation of the largest motor units. Our findings indicate that the patterned modulation of excitability in motor pathways that occurs during rhythmic movements of the opposite limb is mediated primarily by interhemispheric interactions between cortical motor areas.     

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.     

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.     

Modulation of short latency stretch reflexes during human hopping

To gain insight into central and peripheral reflex control mechanisms in moving humans we have investigated short latency stretch reflex activity in m. tricgif surae during two legged hopping. The objectives were: (1) to compare movement induced short latency stretch reflexes in soleus and medial gastrocnemius (MG) muscles, (2) to determine the relationship between the size of these reflexes and the muscle spindle stretch velocities, and (3) to compare the size of the movement induced short latency stretch reflexes and the H-reflexes simultaneously. Six well-trained healthy male subjects participated and they hopped at three different work rates. Surface electromyogram (EMG) and H-reflexes were recorded during hopping. Muscle spindle length changes were estimated as the difference between estimated origin-to-insertion length changes and tendon length changes. The important findings were that during hopping: (1) movement induced short latency stretch reflexes were observed consistently in soleus, (2) the EMG amplitude of this stretch reflex was negatively correlated with the estimated peak muscle spindle stretch velocity (r_s=?0.52, P < 0.02), and (3) the amplitude of the soleus H-reflex at touchdown did not change in parallel with the stretch reflex. The negative correlation observed between the stretch reflex and the estimated peak muscle spindle stretch velocity in soleus is opposite to the basic velocity sensitive behaviour of stretch reflexes mechanically elicited during resting conditions. Possible control mechanisms are discussed. Additionally, muscle spindle length changes estimated from changes in the skeletal movements (joint angles) should be inferred cautiously because of tendon compliance, especially at high tendon forces.     

Short-term plasticity of human spinal inhibitory circuits after isometric and isotonic ankle training

The purpose of this study was to determine to what extent one session of isotonic and isometric ankle dorsi and plantar flexion training induces changes in the frequency-dependent depression of the soleus H-reflex. Further, adaptation of reciprocal Ia inhibition exerted from tibialis anterior flexor group I afferents on soleus motoneurons, and presynaptic inhibition of Ia afferent terminals induced by a conditioning afferent volley following stimulation of the antagonist nerve were established with subjects seated before and after training. The soleus H-reflexes evoked at the inter-stimulus intervals of 1, 2, 3, 5, and 8 s were normalized to the mean amplitude of the H-reflex evoked every 10 s. Conditioned H-reflexes were normalized to the associated control H-reflex evoked with subjects seated before and after training. Twenty-six subjects were randomly assigned to one or more of the 4 exercise groups. Isometric ankle dorsi flexion training decreased the reciprocal and presynaptic inhibition, while isotonic ankle dorsi flexion had no significant effects. Isotonic plantar flexion training decreased only the reciprocal inhibition, whilst isometric plantar flexion had no significant effects on the reciprocal or presynaptic inhibition. None of the training exercise protocols affected the amount of homosynaptic depression of the soleus H-reflex. Our findings support the notion that plastic changes of reciprocal and presynaptic inhibition due to exercise are transferrable to a resting state, and that homosynaptic depression remains unaltered after a single session of ankle training. Further research is needed to outline the time-course of plastic changes of spinal inhibitory mechanisms in humans.     

From activity to rest: gating of excitatory autogenetic afferences from the relaxing muscle in man

Summary The changes in reflex excitability of the motoneurones to the soleus (Sol) muscle occurring during and after voluntary releases of various duration from a constant plantar-torque level in isometric or isotonic conditions have been investigated in normal humans by means of the H-reflex and T-response. The amplitude of both reflexes during the release phase attains values lower than control (obtained in resting conditions) even in the presence of force and EMG activity; the maximal inhibition is reached at the end of the release; the reflexes recover gradually to control values over several seconds. The more abrupt is the release, the more inhibited is the reflex and the shorter is the time to recovery and vv. These results apply both in isometric and isotonic conditions. Activation of the antagonist muscles, sometimes occurring at the end of the fastest release, does not contribute to the H-reflex inhibition. Tonic isometric contractions and relative releases have also been evoked by the tonic vibration reflex (TVR). The H-reflex during the TVR-induced contractions were lower than control values, at variance with those obtained during the voluntary contractions, but their amplitudes during the releases had similar values and time-courses in both conditions, pointing to a common involved inhibitory mechanism. Any voluntary ballistic or ramp contraction taking place after a preceding release, in the period in which the H-reflexes were still inhibited, was not apparently influenced, despite the fact that H-reflexes evoked during the release-conditioned ramp contractions were significantly lower than when evoked during control ramps of similar characteristics. The results are discussed in terms of a premotoneuronal, possibly presynaptic, inhibitory mechanism.Supported by Italian M.P.I.     

Amplitude modulation of the human quadriceps tendon jerk reflex during gait

Summary Amplitude modulation of the quadriceps tendon jerk reflex was investigated during the step cycle in normal human subjects. Reflex amplitude was compared with that obtained during a control stance condition, with equivalent levels of EMG activity and limb position. During gait there was a progressive decrease in the reflex amplitude early in the stance phase, i.e. during yielding of the knee, and it remained reduced throughout the step cycle. This pattern of changes in reflex amplitude correlated with neither the quadriceps EMG activity nor with the knee joint movements. The behavior of the tendon reflex was similar to that described for the modulation of the quadriceps H-reflex during the early stages of the stance phase of gait. In the latter study it was argued that changes in presynaptic inhibition of quadriceps la terminals could account for the amplitude modulation. We conclude that there is no dramatic change in the gamma drive to quadriceps muscle spindles: tendon reflexes are modulated during the step cycle in much the same way as H-reflexes, in spite of the peripheral and central differences between them. Similar behavior has been described for the soleus H-reflex and Achilles tendon reflex during gait although the modulation of these reflexes followed a different pattern than that seen in the quadriceps.     

Effects of hip joint angle changes on intersegmental spinal coupling in human spinal cord injury

Pathological expression of movement and muscle tone in human upper motor neuron disorders has been partly associated with impaired modulation of spinal inhibitory mechanisms, such as reciprocal or presynaptic inhibition. In addition, input from specific afferent systems contributes significantly to spinal reflex circuits coupled with posture or locomotion. Accordingly, the objectives of this study were to identify the involved afferents and their relative contribution to soleus H-reflex modulation induced by changes in hip position, and to relate these effects with activity of spinal interneuronal circuits. Specifically, we investigated the actions of group I synergistic and antagonistic muscle afferents (e.g. common peroneal nerve, CPN; medial gastrocnemius, MG) and tactile plantar cutaneous afferents on the soleus H-reflex during controlled hip angle variations in 11 motor incomplete spinal cord injured (SCI) subjects. It has been postulated in healthy subjects that CPN stimulation evokes an inhibition on the soleus H-reflex at a conditioning test (C-T) interval of 2–4 ms. This short latency reflex depression is caused mainly by activation of the reciprocal Ia inhibitory pathway. At longer C-T intervals (beyond 30 ms) the soleus H-reflex is again depressed, and is generally accepted to be caused by presynaptic inhibition of soleus Ia afferents. Similarly, MG nerve stimulation depresses soleus H-reflex excitability at the C-T interval of 6 ms, involving the pathway of non-reciprocal group I inhibition, while excitation of plantar cutaneous afferents affects the activity of spinal reflex pathways in the extensors. In this study, soleus H-reflexes recorded alone or during CPN stimulation at either short (2, 3, 4 ms) or long (80, 100, 120 ms) C-T intervals, and MG nerve stimulation delivered at 6 ms were elicited via conventional methods and similar to those adopted in studies conducted in healthy subjects. Plantar skin conditioning stimulation was delivered through two surface electrodes placed on the metatarsals at different C-T intervals ranging from 3 to 90 ms. CPN stimulation at either short or long C-T intervals and MG nerve stimulation resulted in a significant facilitation of the soleus H-reflex, regardless of the hip angle tested. Plantar skin stimulation delivered with hip extended at 10° induced a bimodal facilitation reflex pattern, while with hip flexed (10°, 30°) the reflex facilitation increased with increments in the C-T interval. This study provides evidence that in human chronic SCI, classically key inhibitory reflex actions are switched to facilitatory, and that spinal processing of plantar cutaneous sensory input and actions of synergistic/antagonistic muscle afferents interact with hip proprioceptive input to facilitate soleus H-reflex excitability. These actions might be associated with the pathological expression of neural control of movement in individuals with SCI, and potentially could be considered in rehabilitation programs geared to restore sensorimotor function in these patients.     

Stretch and H reflexes in triceps surae are similar during tonic and rhythmic contractions in high decerebrate cats

During locomotion in decerebrate and spinal cats the group Ia afferents from hind leg muscles are depolarized rhythmically. An earlier study concluded that this locomotor-related primary afferent depolarization (PAD) does not contribute to modulation of monosynaptic reflex pathways during locomotion. This finding indicated that the neural network generating the locomotor rhythm, the central pattern generator (CPG), does not presynaptically inhibit monosynaptic reflexes. In this investigation we tested this prediction in decerebrate cats by measuring the magnitude of reflexes evoked in ankle extensor muscles during periods of tonic contractions and during sequences of rhythmic contractions. The latter occurred when the animal was induced to walk on a treadmill. At the similar levels of activity in the soleus muscle there was no significant difference in the magnitude of the soleus H reflex in these two behavioral situations. Similar results were obtained for reflexes evoked by brief stretches of the soleus muscle. We also examined the reflexes evoked by ramp-and-hold stretches during periods of rhythmic and tonic activity of the isolated medial gastrocnemius (MG) muscle. At similar levels of background activity, the reflexes evoked in the MG muscle were the same during rhythmic and tonic contractions. Our failure to observe a reduction in the magnitude of H reflexes and stretch reflexes during rhythmic contractions, compared with reflexes evoked at the same level of background activity during tonic contractions, is consistent with the notion that the CPG for stepping does not presynaptically inhibit monosynaptic reflexes during the extension phase of locomotor activity. Our results indicate that presynaptic inhibition of the monosynaptic reflex associated with normal locomotion in cats or humans arises from sources other than the extensor burst generating system of the central pattern generator.     

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.