Sabata Gervasio

Crossed reflex reversal during human locomotion

During human walking, precise coordination between the two legs is required in order to react promptly to any sudden hazard that could threaten stability. The networks involved in this coordination are not yet completely known, but a direct spinal connection between soleus (SOL) muscles has recently been revealed. For this response to be functional, as previously suggested, we hypothesize that it will be accompanied by a reaction in synergistic muscles, such as gastrocnemius lateralis (GL), and that a reversal of the response would occur when an opposite reaction is required. In the present study, surface EMGs of contralateral SOL and GL were analyzed after tibial nerve (TN), sural nerve (SuN), and medial plantar nerve (MpN) stimulation during two tasks in which opposite reactions are functionally expected: normal walking (NW), just before ipsilateral heel strike, and hybrid walking (HW) (legs walking in opposite directions), at ipsilateral push off and contralateral touchdown. Early crossed facilitations were observed in the contralateral GL after TN stimulation during NW, and a reversal of such responses occurred during HW. These results underline the functional significance of short-latency crossed responses and represent the first evidence for short-latency reflex reversal in the contralateral limb for humans. Muscle afferents seem to mediate the response during NW, while during HW cutaneous afferents are likely involved. It is thus possible that different afferents mediate the crossed response during different tasks.

Sensory feedback in interlimb coordination: Contralateral afferent contribution to the short-latency crossed response during human walking

A constant coordination between the left and right leg is required to maintain stability during human locomotion, especially in a variable environment. The neural mechanisms underlying this interlimb coordination are not yet known. In animals, interneurons located within the spinal cord allow direct communication between the two sides without the need for the involvement of higher centers. These may also exist in humans since sensory feedback elicited by tibial nerve stimulation on one side (ipsilateral) can affect the muscles activation in the opposite side (contralateral), provoking short-latency crossed responses (SLCRs). The current study investigated whether contralateral afferent feedback contributes to the mechanism controlling the SLCR in human gastrocnemius muscle. Surface electromyogram, kinematic and kinetic data were recorded from subjects during normal walking and hybrid walking (with the legs moving in opposite directions). An inverse dynamics model was applied to estimate the gastrocnemius muscle proprioceptors’ firing rate. During normal walking, a significant correlation was observed between the magnitude of SLCRs and the estimated muscle spindle secondary afferent activity (P = 0.04). Moreover, estimated spindle secondary afferent and Golgi tendon organ activity were significantly different (P ≤ 0.01) when opposite responses have been observed, that is during normal (facilitation) and hybrid walking (inhibition) conditions. Contralateral sensory feedback, specifically spindle secondary afferents, likely plays a significant role in generating the SLCR. This observation has important implications for our understanding of what future research should be focusing on to optimize locomotor recovery in patient populations.

Motor adaptation following split-belt treadmill walking.

Malone L, Vasudevan E, and Bastian A (J Neurosci 31: 15136-15143, 2011) investigated the effects of different training paradigms on the day-by-day retention of learned motor patterns. In this Neuro Forum, a description and assessment of the methods used will be presented. The interpretation of the findings will be extended and the possible implications will be discussed. Finally, alternative explanations of the possible regions involved in motor pattern relearning will be provided.

The effect of crossed reflex responses on dynamic stability during locomotion

In recent studies, we demonstrated that a neural pathway within the human spinal cord allows direct communication between muscles located in the opposing limb. Short-latency crossed responses (SLCRs) are elicited in the contralateral triceps surae at an onset of 40-69 ms following electrical stimulation of the ipsilateral tibial nerve (iTN). The SLCRs are significantly affected by lesions of the central nervous system where the patients are unable to attain normal walking symmetry. The aim of this study was to elucidate the functionality of SLCRs by investigating their effects on the center of pressure (CoP) and pressure distribution. SLCRs were elicited by iTN stimulation at the end of the ipsilateral swing phase while the participants (n = 8) walked on a treadmill. CoP location and pressure distribution on the sole of the contralateral foot were recorded using instrumented insoles inserted bilaterally in the participants shoes. The SLCR induced a significant displacement of the CoP toward the medial and anterior direction, associated with a significant increase in pressure at the level of the first metatarsal head. The SLCR contributed to dynamic stability, accelerating the propulsion phase of the contralateral leg and thus preparing for a faster step in the event that the ipsilateral leg is not able to support body weight. The results presented here provide new insight into the functionality of SLCRs, introducing the perspective that training these reflexes, as shown successfully for other reflex pathways, would increase dynamic stability in patients with impaired locomotion.

Replace, Repair, Restore, Relieve : Bridging Clinical and Engineering Solutions in Neurorehabilitation

The present study developed a model that can predict the outcomes of a gait rehabilitation method seen from a societal perspective. The model consisted of a Markov model for the long-term functional dependency of the stroke patients and a decision tree model for the consequences of a stroke. In this study, it was applied for predicting the outcome of introducing reflex-based electrical gait therapy. The model was validated using data for 1 month, 3 months, 1 year, 5 years and 10 years fatality of stroke patients, as well as data for the functional independence of stroke patients. The model predicted that after the first stroke with reflex-based electrical gait therapy, 51.7% of the patients treated will be depending on care and 72.5% will live in their home, as compared with 60.1% and 63.0% for the control group, respectively. After five years, 26.8% of the patients will be depending on care and 34.8% will live in their home following reflex-based gait therapy, as compared with 30.6% and 29.4% for the control group, respectively. Consequently, the study supports the assumption that better gait rehabilitation lead to more functionally independent patients, and hence may lead to lower costs for the society.

Evidence for a supraspinal contribution to the human crossed reflex response during human walking

In humans, an ipsilateral tibial nerve (iTN) stimulation elicits short-latency-crossed-responses (SLCR) comprised of two bursts in the contralateral gastrocnemius lateralis (cGL) muscle. The average onset latency has been reported to be 57–69 ms with a duration of 30.4 ± 6.6 ms. The aim of this study was to elucidate if a transcortical pathway contributes to the SLCR. In Experiment 1 (n = 9), single pulse supra-threshold transcranial magnetic stimulation (supraTMS) was applied alone or in combination with iTN stimulation (85% of the maximum M-wave) while participants walked on a treadmill (delay between the SLCR and the motor evoked potentials (MEP) varied between −30 and 200 ms). In Experiment 2 (n = 6), single pulse sub-threshold TMS (subTMS) was performed and the interstimulus interval (ISI) varied between 0–30 ms. In Experiment 3, somatosensory evoked potentials (SEPs) were recorded during the iTN stimulation to quantify the latency of the resulting afferent volley at the cortical level. SLCRs and MEPs in cGL occurred at 63 ± 6 ms and 29 ± 2 ms, respectively. The mean SEP latency was 30 ± 3 ms. Thus, a transcortical pathway could contribute no earlier than 62–69 ms (SEP+MEP+central-processing-delay) after iTN stimulation. Combined iTN stimulation and supraTMS resulted in a significant MEP extra-facilitation when supraTMS was timed so that the MEP would coincide with the late component of the SLCR, while subTMS significantly depressed this component. This is the first study that demonstrates the existence of a strong cortical control on spinal pathways mediating the SLCR. This likely serves to enhance flexibility, ensuring that the appropriate output is produced in accord with the functional demand.

Technologically-advanced assessment of upper-limb spasticity: a pilot study

BACKGROUND Spasticity is a muscle disorder associated with upper motor neuron syndrome occurring in neurological disorders, such as stroke, multiple sclerosis, spinal cord injury and others. It influences the patients rehabilitation, interfering with function, limiting independence, causing pain and producing secondary impairments, such as contractures or other complications. Due to the heterogeneity of clinical signs of spasticity, there is no agreement on the most appropriate assessment and measurement modality for the evaluation of treatment outcomes. AIM The aim of this article is to propose the use of new robotic devices for upper-limb spasticity assessment and describe the most relevant measures of spasticity which could be automatically assessed by using a technologically advanced device. DESIGN Observational pilot study. SETTING The treatment was provided in a Rehabilitation Centre where the device was located and the subjects were treated in an outpatients setting. POPULATION Five post-stroke patients, age range 19-79 years (mean age 61, standard deviation [SD]±25) in their chronic phase. METHODS A new robotic device able to automatically assess upper-limb spasticity during passive and active mobilization has been developed. The elbow spasticity of five post stroke patients has been assessed by using the new device and by means of the Modified Ashworth Scale (MAS). After the first assessment, subjects were treated with botulin toxin injections, and then underwent 10 sessions of robotic treatments. After the treatment, subjects spasticity was assessed by using the robotic device and the MAS Score. RESULTS In four out of five patients, the botulin toxin injection and robotic treatment resulted in the improvement of the MAS Score; in three patients the robotic measures were able to detect the MAS changes. In one subject botulin toxin was not effective and the robotic device was able to detect the lack of effectiveness. CONCLUSIONS By using the robotic device some spasticity parameters can be continuously recorded during the rehabilitation treatment in order to objectively measure the effectiveness of the interventions provided. CLINICAL REHABILITATION IMPACT The standardized evaluation parameters recorded using robotic devices may provide several advantages: 1) the measures for spasticity assessment can be monitored during every rehabilitation session (even during each movement); 2) these measurements are able to highlight even small changes; 3) the recovery plateau can be detected early thus avoiding further rehabilitation sessions; and 4) these measurements can reduce the assessment bias in multicenter studies.

Exploring the EEG Signatures of Musculoskeletal Pain

Musculoskeletal pain is the most frequent health complaint reported by workers in Europe. Neurofeedback has been proposed to be an alternative to the current treatment of pain, however, the extent to which musculoskeletal pain alters the electroencephalographic (EEG) signal is still not known. The current study aims at identifying the signal characteristics provoked by musculoskeletal pain during movement. Healthy volunteers and patients diagnosed with Lateral Epicondylalgia performed wrist extension movements while EEG signals were collected. The power of the EEG signal was calculated and differences between healthy volunteers and patients were assessed. EEG activity of pain patients differed significantly from that observed in heathy volunteers within the alpha and beta band. This alteration is movement related and is particularly visible in frontal channel locations. The results of the current study are currently being implemented for the development of a neurofeedback protocol to treat musculoskeletal pain.

Motor Control and Emerging Therapies for Improving Mobility in Patients with Spasticity

Treatment of spasticity has traditionally been targeted at reducing stretch reflex activity and muscle tone. However, these spasticity indicators and the actual movement disorder following a spinal or supraspinal lesion have been found to be unrelated. Increased muscle tone could be considered secondary and adaptive to a primary disorder and necessary for the continuing support of the body during locomotion. It is evident that antispastic medication is necessary for patients who experience severe pain and discomfort associated with increased muscle tone during rest. However, most therapies currently prescribed are directed at reducing excitation or increasing inhibition and may potentially interfere with voluntary movement. Impairment of walking may be due to a lack of descending input and a reduction in the afferent input to the spinal neuronal circuits. The result is reduced muscle strength, decreased physiological modulation of reflexes and muscle activity as well as cocontraction of agonist and antagonist muscles. Future therapeutic approaches aiming to assist ambulation in mobile spastic patients should focus on the treatment of these aspects in order to improve a patient’s movement ability. Emerging therapies include robot-assisted treadmill training, repetitive electrical stimulation, paired associative stimulation (PAS) and H-reflex conditioning which may provide a new approach for restoring motor function in the spastic patient population.

Cortical Contribution to Crossed Reflexes in Walking Humans

In this pilot study we investigated whether a transcortical pathway mediates the short-latency responses elicited in the contralateral gastrocnemius lateralis (cGL) by ipsilateral posterior tibial nerve (iPTN) stimulation. For this purpose, single pulse sub-threshold (90% active motor threshold) transcranial magnetic stimulation (subTMS) was applied in combination with iPTN stimulation at several interstimulus intervals (ISI) while the subjects (n=6) walked. SubTMS has been shown to reduce cortical output to the target muscle, inducing a suppression in the EMG signal of the voluntary activated muscle. While no differences were observed for the first component to the response, a considerable depression of the second part, referred to as long-latency component (LLC) was observed when the subTMS was timed such that the induced suppression would coincide with this component. These results strongly suggest the cortical control of the LLC in the cGL response. This allows for an enhanced flexibility during unexpected perturbations, ensuring that the appropriate output is produced in accord with the functional demand.