Reflexes from the superficial peroneal nerve during walking in stroke subjects

Cutaneous reflexes from the foot during gait in hereditary spastic paraparesis

OBJECTIVE

It is known that P2 cutaneous reflexes from the foot show phase-dependent modulation during gait. The role of the motor cortex and the cortico-spinal tract in these reflexes and their modulation is unknown. Patients with hereditary spastic paraparesis (HSP) have a lesion in the cortico-spinal tract and may show deficits in P2 reflexes and/or their modulation.

METHODS

Reflex responses of tibialis anterior and biceps femoris after sural nerve stimulation in 10 HSP-patients were compared with those in 10 healthy subjects. The reflexes were studied at two different moments in the step cycle during walking on a treadmill.

RESULTS

Both patients and controls showed a phase-dependent modulation of P2 responses. For the individual muscles, no significant difference in reflex activity was observed between HSP-patients and the controls. However, when all muscles were taken together, the reflex activity for the controls was significantly higher than for the patients.

CONCLUSIONS

The results of this study suggest that the cortico-spinal tract is involved in the regulation of the amplitude of the P2 responses and their phase-dependent modulation.

Direction-dependent phasing of locomotor muscle activity is altered post-stroke

A major contributor to impaired locomotion post-stroke is abnormal phasing of muscle activity. While inappropriate paretic muscle phasing adapts to changing body orientation, load, and speed, it remains unclear whether paretic muscle phasing adapts to reversal of locomotor direction. We examined muscle phasing in backward pedaling, a task that requires shifts in biarticular but not uniarticular muscle phasing relative to forward pedaling. We hypothesized that if paretic and neurologically intact muscle phasing adapt similarly, then paretic biarticular but not paretic uniarticular muscles would shift phasing in backward pedaling. Paretic and neurologically intact individuals pedaled forward and backward while recording electromyograms (EMGs) from vastus medialis (VM), soleus (SOL), rectus femoris (RF), semimembranosus (SM), and biceps femoris (BF). Changes in muscle phasing were assessed by comparing the probability of muscle activity in forward and backward pedaling throughout 18 pedaling cycles. Paretic uniarticular muscles (VM and SOL) showed phase-advanced activity in backward versus forward pedaling, whereas the corresponding neurologically intact muscles showed little to no phasing change. Paretic biarticular muscles were less likely than neurologically intact biarticular muscles to display phasing changes in backward pedaling. Paretic RF displayed no phase change during backward pedaling, and paretic BF displayed no consistent adaptation to backward pedaling. Paretic SM was the only muscle to display backward/forward phase changes that were similar to the neurologically intact group. We conclude that paretic uniarticular muscles are more susceptible and paretic biarticular muscles are less susceptible to direction-dependent phase shifts, consistent with altered sensory integration and impaired cortical control of locomotion.

Reflex receptive fields for human withdrawal reflexes elicited by non-painful and painful electrical stimulation of the foot sole

OBJECTIVES

Human withdrawal reflex receptive fields (RRFs) were assessed for 4 different electrical stimulus intensities, ranging from below the pain threshold (PTh) to up to two times the PTh intensity (0.8x, 1.2x, 1.6x, and 2.0xPTh).

METHODS

Thirteen subjects participated, and the reflexes were recorded in a sitting position. The stimuli were delivered in random order to 12 positions distributed over the foot sole. Tibialis anterior (TA), gastrocnemius medialis (GM), vastus lateralis (VL), and biceps femoris (BF) reflexes were recorded. Further, knee and ankle joint angle changes were recorded.

RESULTS

The strongest reflexes were seen in the TA compared with the other 3 muscles. Dorsi-flexion dominated distal to the talocrural joint corresponding to the TA receptive field area. An expansion of the RRF for the TA and GM was seen when increasing the stimulus intensity from 0.8xPTh to 1.2xPTh and from 1.2xPTh to 1.6xPTh, indicating a gradually increasing reflex threshold towards the border, where TA contraction is inappropriate in a withdrawal reaction. For the BF and VL, the borders of the RRF areas were not detected. By integrating the reflex size within the RRF (i.e. the reflex volume), gradually increasing reflexes for increasing stimulus intensity were seen in all 4 muscles tested, most clearly in the TA and GM. The subjective pain intensity correlated to the reflex volume for the TA, GM, and BF.

CONCLUSIONS

In conclusion, the highest reflex sensitivity was seen in the centre of the RRF, while the stimulus intensity needed for eliciting a reflex increased towards the receptive field border. Within the RRF, stronger reflexes were evoked for increasing stimulus intensity. The limit in the size of the receptive field size for the TA and GM supports a modular withdrawal reflex organisation.

Differential regulation of cutaneous and H-reflexes during leg cycling in humans

Reflexes undergo modulation according to task and timing during standing, walking, running, and leg cycling in humans. Both cutaneous and Hoffman (H-) reflexes are modulated by movement and task. However, recent evidence suggests that the modulation pattern for cutaneous and H-reflexes may be different. We sought to clarify this issue by reducing the effect of movement phase and altering the level of background muscle activation (low and high) in static and dynamic (leg cycling) conditions. Electromyography was recorded from the ankle extensors soleus and medial gastrocnemius (MG) and the knee extensor vastus lateralis (VL). Reflexes were evoked during the downstroke of stationary leg cycling. Cutaneous reflexes were evoked with trains of 5 x 1.0 ms pulses at 300 Hz delivered to the distal tibial nerve, whereas H-reflexes were evoked in soleus by stimulation with single 1.0-ms pulses. There were two main observations in this study: 1) middle latency cutaneous reflexes were facilitatory during static contraction but were dramatically attenuated or reversed to suppressive responses during cycling (task-dependent modulation); 2) soleus H-reflexes were larger in the high muscle activation condition but were unaffected by task (no task-dependent modulation). Thus opposite results were obtained in the two reflex pathways. It is concluded that cutaneous and H-reflexes are modulated by different mechanisms during active locomotor-like movements.

Task-related changes of transmission in the pathway of heteronymous spinal recurrent inhibition from soleus to quadriceps motor neurones in man

An H reflex conditioning technique was used to monitor the transmission of heteronymous recurrent inhibition from soleus to quadriceps motor neurones of the human lower limb. Inhibition declined during quadriceps muscle contraction under all conditions examined, falling to zero at around one-third of the maximum voluntary contraction. Inhibition declined during soleus muscle contraction in sitting, standing and bicycling tasks. The level of inhibition assessed at a given (weaker than 30%) level of quadriceps contraction was reduced during postural tasks involving quadriceps and soleus co-contraction (standing and late-stance phase of walking) when compared with sitting and performing matched voluntary muscle contractions. The level of inhibition during the mid-power stroke of a bicycling task, which also involved co-contraction of quadriceps and soleus, was greater than during matched voluntary muscle contractions while sitting. It is concluded that the pathway of heteronymous recurrent inhibition from soleus to quadriceps motor neurones is under at least two types of control: one related to the task, which sets the operating range, and a second which couples inhibition to the level of muscle contraction. Multiple control pathways are consistent with the diverse effects on recurrent inhibition reported in subjects with upper motor neurone lesions.

Modulation of human cutaneous reflexes during rhythmic cyclical arm movement

The organization and pattern of cutaneous reflex modulation is unknown during rhythmic cyclical movements of the human upper limbs. On the assumption that these cyclic arm movements are central pattern generator (CPG) driven as has been suggested for leg movements such as walking, we hypothesized that cutaneous reflex amplitude would be independent of electromyographic (EMG) muscle activation level during rhythmic arm movement (phase-dependent modulation, as is often the case in the lower limb during locomotion). EMG was recorded from eight muscles crossing the human shoulder, elbow, and wrist joints while whole arm rhythmic cyclical movements were performed. Cutaneous reflexes were evoked with trains of electrical stimulation delivered at non-noxious intensities (approximately 2 x threshold for radiating paresthesia) to the superficial radial nerve innervating the lateral portion of the back of the hand. Phasic bursts of rhythmic muscle activity occurred throughout the movement cycle. Rhythmic EMG and kinematic patterns were similar to what has been seen in the human lower limb during locomotor activities such as cycling or walking: there were extensive periods of reciprocal activation of antagonist muscles. For most muscles, cutaneous reflexes were modulated with the movement cycle and were strongly correlated with the movement-related background EMG amplitude. It is concluded that cutaneous reflexes are primarily modulated by the background muscle activity during rhythmic human upper limb movements, with only some muscles showing phase-dependent modulation.

Evidence for transcortical reflex pathways in the lower limb of man

The existence of transcortical reflex pathways in the control of distal arm and hand muscles in man is now widely accepted. Much more controversy exists regarding a possible contribution of such reflexes to the control of leg muscles. It is often assumed that transcortical reflex pathways play no, or only a minor, role in the control of leg muscles. Transcortical reflex pathways according to this view are reserved for the control of the distal upper limb and are seen in close relation to the evolution of the primate hand. Here we review data, which provide evidence that transcortical reflexes do exist for lower limb muscles and may play a significant role in the control of at least some of these muscles. This evidence is based on animal research, recent experiments combining transcranial magnetic stimulation with peripheral electrical and mechanical stimulation in healthy subjects and neurological patients. We propose that afferent activity from muscle and skin may play a role in the regulation of bipedal gait through transcortical pathways.

Abeta fibers mediate cutaneous reflexes during human walking

During human gait, transmission of cutaneous reflexes from the foot is controlled specifically according to the phase of the step cycle. These reflex responses can be evoked by nonnociceptive stimuli, and therefore it is thought that the large-myelinated and low-threshold Abeta afferent fibers mediate these reflexes. At present, this hypothesis is not yet verified. To test whether Abeta fibers are involved the reflex responses were studied in patients with a sensory polyneuropathy who suffer from a predominant loss of large-myelinated Abeta fibers. The sural nerve of both patients and healthy control subjects was stimulated electrically at a nonnociceptive intensity during the early and late swing phases while they walked on a treadmill. The responses were studied by recording electromyographic (EMG) activity of the biceps femoris (BF) and tibialis anterior (TA) of the stimulated leg. In both phases, large facilitatory responses were observed in the BF of the healthy subjects. These facilitations were reduced significantly in the BF of the patients, indicating that Abeta fibers mediate these reflexes. In TA similar results were obtained. The absolute response magnitude across the two phases was significantly smaller for the patients than for the healthy subjects. The TA responses for the healthy subjects were on average facilitatory during early swing and suppressive during end swing. Both facilitations and suppressions were considerably smaller for the patients, indicating that both types of responses are mediated by Abeta fibers. It is concluded that low-threshold Abeta sensory fibers mediate these reflexes during human gait. The low threshold and the precise phase-dependent control of these responses suggest that these responses are important in the regulation of gait. The loss of such reflex activity may be related to the gait impairments of these patients.

What functions do reflexes serve during human locomotion?

Studies on the reflex modulation of vertebrate locomotion have been conducted in many different laboratories and with many different preparations: for example, lamprey swimming, bird flight, quadrupedal walking in cats and bipedal walking in humans. Emerging concepts are that reflexes are task-, phase- and context-dependent. To function usefully in a behaviour such as locomotion wherein initial conditions change from step to step, reflexes would have to show modulation. Papers are reviewed in which the study of different reflexes have been conducted during different behaviours, with an emphasis on experiments in humans. A framework is developed in which the modulation and flexibility of reflexes are demonstrated. Alterations in cutaneous, and muscle (stretch and load receptor) reflexes between sitting, standing and walking are discussed. Studies in which both electrical, mechanical and 'natural' receptor activation have been conducted during walking are reviewed. Reflexes are shown to have important regulatory functions during human locomotion. A framework for discussion of reflex function throughout the step cycle is developed. The function of a given reflex pathway changes dynamically throughout the locomotor cycle. While all reflexes act in concert to a certain extent, generally cutaneous reflexes act to alter swing limb trajectory to avoid stumbling and falling. Stretch reflexes act to stabilize limb trajectory and assist force production during stance. Load receptor reflexes are shown to have an effect on both stance phase body weight support and step cycle timing. After neurotrauma or in disease, reflexes no longer function as during normal locomotion, but still have the potential to be clinically exploited in gait modification regimens.