Evidence for a spinal stepping generator in man

Epidural spinal cord stimulation for motor recovery in spinal cord injury: A systematic review

BACKGROUND

Epidural spinal cord stimulation (ESCS) emerged as a technology for eliciting motor function in the 1990's and was subsequently employed therapeutically in the population with spinal cord injury (SCI). Despite a considerable number of ESCS studies, a comprehensive systematic review of ESCS remains unpublished.

OBJECTIVE

The current review of the existing literature evaluated the efficacy of ESCS for improving motor function in individuals with SCI.

METHODS

A search for ESCS studies was performed using the following databases: Medline (Ovid), Web of Science and Embase. Furthermore, to maximize results, an inverse manual search of references cited by identified articles was also performed. Studies published between January 1995 and June 2020 were included. The search was constructed around the following key terms: Spinal cord stimulation, SCI and motor response generation.

RESULTS

A total of 3435 articles were initially screened, of which 18 met the inclusion criteria. The total sample comprised of 24 participants with SCI. All studies reported some measure of improvement in motor activity with ESCS, with 17 reporting altered EMG responses. Functional improvements were reported in stepping (n = 11) or muscle force (n = 4). Only 5 studies assessed ASIA scale pre- and post-intervention, documenting improved classification in 4 of 11 participants. Appraisal using the modified Downs and Black quality checklist determined that reviewed studies were of poor quality. Due to heterogeneity of outcome measures utilized in studies reviewed, a meta-analysis of data was not possible.

CONCLUSION

While the basic science is encouraging, the therapeutic efficacy of ESCS remains inconclusive.

Spinal Cord Injury Alters Spinal Shox2 Interneurons by Enhancing Excitatory Synaptic Input and Serotonergic Modulation While Maintaining Intrinsic Properties in Mouse

Neural circuitry generating locomotor rhythm and pattern is located in the spinal cord. Most spinal cord injuries (SCIs) occur above the level of spinal locomotor neurons; therefore, these circuits are a target for improving motor function after SCI. Despite being relatively intact below the injury, locomotor circuitry undergoes substantial plasticity with the loss of descending control. Information regarding cell type-specific plasticity within locomotor circuits is limited. Shox2 interneurons (INs) have been linked to locomotor rhythm generation and patterning, making them a potential therapeutic target for the restoration of locomotion after SCI. The goal of the present study was to identify SCI-induced plasticity at the level of Shox2 INs in a complete thoracic transection model in adult male and female mice. Whole-cell patch-clamp recordings of Shox2 INs revealed minimal changes in intrinsic excitability properties after SCI. However, afferent stimulation resulted in mixed excitatory and inhibitory input to Shox2 INs in uninjured mice which became predominantly excitatory after SCI. Shox2 INs were differentially modulated by serotonin (5-HT) in a concentration-dependent manner in uninjured conditions but following SCI, 5-HT predominantly depolarized Shox2 INs. 5-HT7 receptors mediated excitatory effects on Shox2 INs from both uninjured and SCI mice, but activation of 5-HT2B/2C receptors enhanced excitability of Shox2 INs only after SCI. Overall, SCI alters sensory afferent input pathways to Shox2 INs and 5-HT modulation of Shox2 INs to enhance excitatory responses. Our findings provide relevant information regarding the locomotor circuitry response to SCI that could benefit strategies to improve locomotion after SCI.SIGNIFICANCE STATEMENT Current therapies to gain locomotor control after spinal cord injury (SCI) target spinal locomotor circuitry. Improvements in therapeutic strategies will require a better understanding of the SCI-induced plasticity within specific locomotor elements and their controllers, including sensory afferents and serotonergic modulation. Here, we demonstrate that excitability and intrinsic properties of Shox2 interneurons, which contribute to the generation of the locomotor rhythm and pattering, remain intact after SCI. However, SCI induces plasticity in both sensory afferent pathways and serotonergic modulation, enhancing the activation and excitation of Shox2 interneurons. Our findings will impact future strategies looking to harness these changes with the ultimate goal of restoring functional locomotion after SCI.

Predictors of volitional motor recovery with epidural stimulation in individuals with chronic spinal cord injury

Spinal cord epidural stimulation (scES) has enabled volitional lower extremity movements in individuals with chronic and clinically motor complete spinal cord injury and no clinically detectable brain influence. The aim of this study was to understand whether the individuals' neuroanatomical characteristics or positioning of the scES electrode were important factors influencing the extent of initial recovery of lower limb voluntary movements in those with clinically motor complete paralysis. We hypothesized that there would be significant correlations between the number of joints moved during attempts with scES prior to any training interventions and the amount of cervical cord atrophy above the injury, length of post-traumatic myelomalacia and the amount of volume coverage of lumbosacral enlargement by the stimulation electrode array. The clinical and imaging records of 20 individuals with chronic and clinically motor complete spinal cord injury who underwent scES implantation were reviewed and analysed using MRI and X-ray integration, image segmentation and spinal cord volumetric reconstruction techniques. All individuals that participated in the scES study (n = 20) achieved, to some extent, lower extremity voluntary movements post scES implant and prior to any locomotor, voluntary movement or cardiovascular training. The correlation results showed that neither the cross-section area of spinal cord at C3 (n = 19, r = 0.33, P = 0.16) nor the length of severe myelomalacia (n = 18, r = -0.02, P = 0.93) correlated significantly with volitional lower limb movement ability. However, there was a significant, moderate correlation (n = 20, r = 0.59, P = 0.006) between the estimated percentage of the lumbosacral enlargement coverage by the paddle electrode as well as the position of the paddle relative to the maximal lumbosacral enlargement and the conus tip (n = 20, r = 0.50, P = 0.026) with the number of joints moved volitionally. These results suggest that greater coverage of the lumbosacral enlargement by scES may improve motor recovery prior to any training, possibly because of direct modulatory effects on the spinal networks that control lower extremity movements indicating the significant role of motor control at the level of the spinal cord.

Recent Insights into the Rhythmogenic Core of the Locomotor CPG

In order for locomotion to occur, a complex pattern of muscle activation is required. For more than a century, it has been known that the timing and pattern of stepping movements in mammals are generated by neural networks known as central pattern generators (CPGs), which comprise multiple interneuron cell types located entirely within the spinal cord. A genetic approach has recently been successful in identifying several populations of spinal neurons that make up this neural network, as well as the specific role they play during stepping. In spite of this progress, the identity of the neurons responsible for generating the locomotor rhythm and the manner in which they are interconnected have yet to be deciphered. In this review, we summarize key features considered to be expressed by locomotor rhythm-generating neurons and describe the different genetically defined classes of interneurons which have been proposed to be involved.

Ipsi- and Contralateral Oligo- and Polysynaptic Reflexes in Humans Revealed by Low-Frequency Epidural Electrical Stimulation of the Lumbar Spinal Cord

Epidural electrical stimulation (EES) applied over the human lumbosacral spinal cord provides access to afferent fibers from virtually all lower-extremity nerves. These afferents connect to spinal networks that play a pivotal role in the control of locomotion. Studying EES-evoked responses mediated through these networks can identify some of their functional components. We here analyzed electromyographic (EMG) responses evoked by low-frequency (2–6 Hz) EES derived from eight individuals with chronic, motor complete spinal cord injury. We identified and separately analyzed three previously undescribed response types: first, crossed reflexes with onset latencies of ~55 ms evoked in the hamstrings; second, oligosynaptic reflexes within 50 ms post-stimulus superimposed on the monosynaptic posterior root-muscle reflexes in the flexor muscle tibialis anterior, but with higher thresholds and no rate-sensitive depression; third, polysynaptic responses with variable EMG shapes within 50–450 ms post-stimulus evoked in the tibialis anterior and triceps surae, some of which demonstrated consistent changes in latencies with graded EES. Our observations suggest the activation of commissural neurons, lumbar propriospinal interneurons, and components of the late flexion reflex circuits through group I and II proprioceptive afferent inputs. These potential neural underpinnings have all been related to spinal locomotion in experimental studies.

Spinal cord stimulation and rehabilitation in an individual with chronic complete L1 paraplegia due to a conus medullaris injury: motor and functional outcomes at 18 months

INTRODUCTION

Epidural electrical stimulation of the conus medullaris has helped facilitate native motor recovery in individuals with complete cervicothoracic spinal cord injuries (SCI). A theorized mechanism of clinical improvement includes supporting central pattern generators intrinsic to the conus medullaris. Because spinal cord stimulators (SCS) are approved for the treatment of neuropathic pain, we were able to test this experimental therapy in a subject with complete L1 paraplegia and neuropathic genital pain due to a traumatic conus injury.

CASE PRESENTATION

An otherwise healthy 48-year-old male with chronic complete L1 paraplegia with no zones of partial preservation (ZPP) and intractable neuropathic genital pain presented to our group seeking nonmedical pain relief and any possible help with functional restoration. After extensive evaluation, discussion, and consent, we proceeded with SCS implantation at the conus and an intensive outpatient physical therapy regimen consistent with the recent SCI rehabilitation literature.

DISCUSSION

Intraoperatively, no electromyography (EMG) could be elicited with epidural conus stimulation. At 18 months after implantation, his motor ZPPs had advanced from L1 to L5 on the left and from L1 to L3 on the right. Qualitative increases in lower extremity resting state EMG amplitudes were noted, although there was no consistent evidence of voluntary EMG or rhythmic locomotive leg movements. Three validated functional and quality of life (QoL) surveys demonstrated substantial improvements. The modest motor response compared to the literature suggests likely critical differences in the anatomy of such a low injury. However, the change in ZPPs and QoL suggest potential for neuroplasticity even in this patient population.

Ambulation in Dogs With Absent Pain Perception After Acute Thoracolumbar Spinal Cord Injury

Acute thoracolumbar spinal cord injury (SCI) is common in dogs frequently secondary to intervertebral disc herniation. Following severe injury, some dogs never regain sensory function to the pelvic limbs or tail and are designated chronically "deep pain negative." Despite this, a subset of these dogs develop spontaneous motor recovery over time including some that recover sufficient function in their pelvic limbs to walk independently without assistance or weight support. This type of ambulation is commonly known as "spinal walking" and can take up to a year or more to develop. This review provides a comparative overview of locomotion and explores the physiology of locomotor recovery after severe SCI in dogs. We discuss the mechanisms by which post-injury plasticity and coordination between circuitry contained within the spinal cord, peripheral sensory feedback, and residual or recovered supraspinal connections might combine to underpin spinal walking. The clinical characteristics of spinal walking are outlined including what is known about the role of patient or injury features such as lesion location, timeframe post-injury, body size, and spasticity. The relationship between the emergence of spinal walking and electrodiagnostic and magnetic resonance imaging findings are also discussed. Finally, we review possible ways to predict or facilitate recovery of walking in chronically deep pain negative dogs. Improved understanding of the mechanisms of gait generation and plasticity of the surviving tissue after injury might pave the way for further treatment options and enhanced outcomes in severely injured dogs.

Independent community walking after a short protocol of repetitive transcranial magnetic stimulation associated with body weight-support treadmill training in a patient with chronic spinal cord injury: a case report

PURPOSE

Our report describes the effect of repetitive transcranial magnetic stimulation (rTMS) combined with body weight-supported treadmill training (BWSTT) on independent gait recovery in a patient with incomplete spinal cord injury (iSCI).

CASE DESCRIPTION

The patient was a 31-year-old male, household ambulator (aid walker) and community wheelchair user who was 8.5 year post traumatic iSCI (T8 vertebra injury, AIS D).

INTERVENTION

The patient participated in 12 sessions (three times/week for four weeks) of rTMS (1800 pulses, 10 Hz, intensity of 90% resting motor threshold) followed by BWSTT (15-20 min, moderate intensity).

OUTCOMES

After treatment, the patient's score increased 3 points on the Walking Index for Spinal Cord Injury II (walking independence) and he became a community ambulator with crutches. His American Spinal Injury Association (ASIA) lower extremities motor score (motor function) increased from 33 to 45 points and the Spinal Cord Independence Measure III (functional independence) score increased from 23 to 29 for the mobility indoors/outdoors subscale. The patient's lower limb spasticity was reduced (Modified Ashworth Scale), and quality of life improved based on the Short-Form Health Survey - 36, and the Patient Global Impression of Change Scale showed considerable perception of improvement.

CONCLUSION

Our report suggests that a short protocol of rTMS combined with BWSTT improved walking independence, motor function, spasticity, functional mobility and quality of life in this patient with iSCI.

Spinal Control of Locomotion: Individual Neurons, Their Circuits and Functions

Systematic research on the physiological and anatomical characteristics of spinal cord interneurons along with their functional output has evolved for more than one century. Despite significant progress in our understanding of these networks and their role in generating and modulating movement, it has remained a challenge to elucidate the properties of the locomotor rhythm across species. Neurophysiological experimental evidence indicates similarities in the function of interneurons mediating afferent information regarding muscle stretch and loading, being affected by motor axon collaterals and those mediating presynaptic inhibition in animals and humans when their function is assessed at rest. However, significantly different muscle activation profiles are observed during locomotion across species. This difference may potentially be driven by a modified distribution of muscle afferents at multiple segmental levels in humans, resulting in an altered interaction between different classes of spinal interneurons. Further, different classes of spinal interneurons are likely activated or silent to some extent simultaneously in all species. Regardless of these limitations, continuous efforts on the function of spinal interneuronal circuits during mammalian locomotion will assist in delineating the neural mechanisms underlying locomotor control, and help develop novel targeted rehabilitation strategies in cases of impaired bipedal gait in humans. These rehabilitation strategies will include activity-based therapies and targeted neuromodulation of spinal interneuronal circuits via repetitive stimulation delivered to the brain and/or spinal cord.

Trunk muscle activation during movement with a new exercise device for lumbo-pelvic reconditioning

Gravitational unloading leads to adaptations of the human body, including the spine and its adjacent structures, making it more vulnerable to injury and pain. The Functional Re‐adaptive Exercise Device (FRED) has been developed to activate the deep spinal muscles, lumbar multifidus (LM) and transversus abdominis (TrA), that provide inter‐segmental control and spinal protection. The FRED provides an unstable base of support and combines weight bearing in up‐right posture with side alternating, elliptical leg movements, without any resistance to movement. The present study investigated the activation of LM, TrA, obliquus externus (OE), obliquus internus (OI), abdominis, and erector spinae (ES) during FRED exercise using intramuscular fine‐wire and surface EMG. Nine healthy male volunteers (27 ± 5 years) have been recruited for the study. FRED exercise was compared with treadmill walking. It was confirmed that LM and TrA were continually active during FRED exercise. Compared with walking, FRED exercise resulted in similar mean activation of LM and TrA, less activation of OE, OI, ES, and greater variability of lumbo‐pelvic muscle activation patterns between individual FRED/gait cycles. These data suggest that FRED continuously engages LM and TrA, and therefore, has the potential as a stationary exercise device to train these muscles.

The Relationship between Trans-Lesional Conduction, Motor Neuron Pool Excitability, and Motor Function in Dogs with Incomplete Recovery from Severe Spinal Cord Injury

Spontaneous, acute, complete thoracolumbar spinal cord injury (TL-SCI) in dogs frequently results in permanent deficits modeling chronic paralysis in people. Recovery of walking without recovery of sensation has been interpreted in dogs as reflexive spinal walking. To evaluate this assumption, this study characterized the electrophysiological status of motor and sensory long tracts and local reflex circuitry in dogs with absent recovery of sensation after acute TL-SCI and correlated findings to gait scores. Twenty dogs with permanent deficits after acute, clinically complete TL-SCI and 6 normal dogs were prospectively enrolled. Transcranial magnetic motor evoked potentials (MEPs), somatosensory evoked potentials (SSEPs), H-reflex, and F-waves were evaluated. Gait was quantified using an ordinal, open field scale (OFS) and treadmill-based stepping and coordination scores (SS, RI). MEP latency and H-reflex variables were compared between cases and controls. Associations between presence of MEPs, SSEPs, F-waves or H-reflex variables, and gait scores were determined. Pelvic limb MEPs were detected in 4 cases; no case had trans-lesional sensory conduction. Latency was longer and conduction velocity slower in cases than controls (p_a = 0.0064, 0.0023, respectively). Three of 4 cases with pelvic limb MEPs were ambulatory, and gait scores (OFS, SS, RI) were each associated with presence of trans-lesional conduction (p_a = 0.006, 0.006, 0.003, respectively). H threshold in cases (mean, 3.2mA ±2.5) was lower than controls (mean, 7.9mA ±3.1; p_a = 0.011) and was inversely associated with treadmill-based scores, SS, and RI (p_a = 0.042, 0.043, respectively). The association between pelvic limb MEPs and gait scores supports the importance of descending influence on regaining walking after severe TL-SCI in dogs rather than just activation of spinal walking. The inverse association between H-reflex threshold and gait scores implies that increases in motor neuron pool excitability might also contribute to motor recovery.

Enabling Task-Specific Volitional Motor Functions via Spinal Cord Neuromodulation in a Human With Paraplegia

We report a case of chronic traumatic paraplegia in which epidural electrical stimulation (EES) of the lumbosacral spinal cord enabled (1) volitional control of task-specific muscle activity, (2) volitional control of rhythmic muscle activity to produce steplike movements while side-lying, (3) independent standing, and (4) while in a vertical position with body weight partially supported, voluntary control of steplike movements and rhythmic muscle activity. This is the first time that the application of EES enabled all of these tasks in the same patient within the first 2 weeks (8 stimulation sessions total) of EES therapy.

How we Walk: Central Control of Muscle Activity during Human Walking

Recent advances in noninvasive electrophysiological and brain imaging techniques have made investigation of the central control of human walking possible. We are thus now able to ask in what way the motor control circuitries in the human brain and spinal cord have been modified in order to control bipedal walking. This information is of importance not only for our understanding of basic control strategies and paradigms but also for future attempts at rehabilitating the gait ability of patients after lesions of the brain and spinal cord.

Interlimb coordination in body-weight supported locomotion: A pilot study

Locomotion involves complex neural networks responsible for automatic and volitional actions. During locomotion, motor strategies can rapidly compensate for any obstruction or perturbation that could interfere with forward progression. In this pilot study, we examined the contribution of interlimb pathways for evoking muscle activation patterns in the contralateral limb when a unilateral perturbation was applied and in the case where body weight was externally supported. In particular, the latency of neuromuscular responses was measured, while the stimulus to afferent feedback was limited. The pilot experiment was conducted with six healthy young subjects. It employed the MIT-Skywalker (beta-prototype), a novel device intended for gait therapy. Subjects were asked to walk on the split-belt treadmill, while a fast unilateral perturbation was applied mid-stance by unexpectedly lowering one side of the split-treadmill walking surfaces. Subject's weight was externally supported via the body-weight support system consisting of an underneath bicycle seat and the torso was stabilized via a loosely fitted chest harness. Both the weight support and the chest harness limited the afferent feedback. The unilateral perturbations evoked changes in the electromyographic activity of the non-perturbed contralateral leg. The latency of all muscle responses exceeded 100ms, which precludes the conjecture that spinal cord alone is responsible for the perturbation response. It suggests the role of supraspinal or midbrain level pathways at the inter-leg coordination during gait.

Oscillations in the human brain during walking execution, imagination and observation

Gait is an essential human activity which organizes many functional and cognitive behaviors. The biomechanical constraints of bipedalism implicating a permanent control of balance during gait are taken into account by a complex dialog between the cortical, subcortical and spinal networks. This networking is largely based on oscillatory coding, including changes in spectral power and phase-locking of ongoing neural activity in theta, alpha, beta and gamma frequency bands. This coding is specifically modulated in actual gait execution and representation, as well as in contexts of gait observation or imagination. A main challenge in integrative neuroscience oscillatory activity analysis is to disentangle the brain oscillations devoted to gait control. In addition to neuroimaging approaches, which have highlighted the structural components of an extended network, dynamic high-density EEG gives non-invasive access to functioning of this network. Here we revisit the neurophysiological foundations of behavior-related EEG in the light of current neuropsychological theoretic frameworks. We review different EEG rhythms emerging in the most informative paradigms relating to human gait and implications for rehabilitation strategies.

Strategies for spinal cord repair after injury: a review of the literature and information

INTRODUCTION

Thanks to the Internet, we can now have access to more information about spinal cord repair. Spinal cord injured (SCI) patients request more information and hospitals offer specific spinal cord repair medical consultations.

OBJECTIVE

Provide practical and relevant elements to physicians and other healthcare professionals involved in the care of SCI patients in order to provide adequate answers to their questions.

METHOD

Our literature review was based on English and French publications indexed in PubMed and the main Internet websites dedicated to spinal cord repair.

RESULTS

A wide array of research possibilities including notions of anatomy, physiology, biology, anatomopathology and spinal cord imaging is available for the global care of the SCI patient. Prevention and repair strategies (regeneration, transplant, stem cells, gene therapy, biomaterials, using sublesional uninjured spinal tissue, electrical stimulation, brain/computer interface, etc.) for the injured spinal cord are under development. It is necessary to detail the studies conducted and define the limits of these new strategies and benchmark them to the realistic medical and rehabilitation care available to these patients.

CONCLUSION

Research is quickly progressing and clinical trials will be developed in the near future. They will have to answer to strict methodological and ethical guidelines. They will first be designed for a small number of patients. The results will probably be fragmented and progress will be made through different successive steps.

Higher level gait disorders in subcortical chronic vascular encephalopathy: a single photon emission computed tomography study

BACKGROUND

the so-called higher level gait disorders include several types of gait disorders in which there are no major modifications in strength, tone, sensitivity, coordination and balance. Brain activation sites related to walking have been investigated using SPECT in humans. The aim of the study was to investigate brain activation during walking in subjects with high-level gait disorders due to chronic subcortical vascular encephalopathy.

SUBJECTS

twelve patients with a chronic vascular encephalopathy were enrolled in the study. Seven subjects had apraxic gait while in the other five the gait was normal. All patients had undergone a recent cerebral magnetic resonance that revealed diffused chronic ischemic lesions within the white matter.

METHODS

all 12 patients underwent a regional cerebral blood flow (rCBF) brain SPECT study with (99m)Tc-Bicisate on two separate days and under two different conditions: at rest (baseline) and while walking (functional).

RESULTS

the rCBF increase induced by the treadmill test (functional-baseline), bilaterally in the medial frontal gyrus and in the anterior lobes of the cerebellum, resulted significantly (P < 0.001) lower in patients with gait apraxia versus those without it.

CONCLUSIONS

this study of the brain with SPECT records the areas of perfusion deficit that appear in apraxic subjects when they walk, compared with the recordings obtained with the same investigation performed at rest.

Spinal myoclonus after spinal cord injury

BACKGROUND/OBJECTIVE

In the course of examining spinal motor function in many hundreds of people with traumatic spinal cord injury, we encountered 6 individuals who developed involuntary and rhythmic contractions in muscles of their legs. Although there are many reports of unusual muscle activation patterns associated with different forms of myoclonus, we believe that certain aspects of the patterns seen with these 6 subjects have not been previously reported. These patterns share many features with those associated with a spinal central pattern generator for walking.

METHODS

Subjects in this case series had a history of chronic injury to the cervical spinal cord, resulting in either complete (ASIA A; n = 4) or incomplete (ASIA D; n = 2) quadriplegia. We used multi-channel electromyography recordings of trunk and leg muscles of each subject to document muscle activation patterns associated with different postures and as influenced by a variety of sensory stimuli.

RESULTS

Involuntary contractions spanned multiple leg muscles bilaterally, sometimes including weak abdominal contractions. Contractions were smooth and graded and were highly reproducible in rate for a given subject (contraction rates were 0.3-0.5 Hz). These movements did not resemble the brief rapid contractions (ie, "jerks") ascribed to some forms of spinal myoclonus. For all subjects, the onset of involuntary muscle contraction was dependent upon hip angle; contractions did not occur unless the hips (and knees) were extended (ie, subjects were supine). In the 4 ASIA A subjects, contractions occurred simultaneously in all muscles (agonists and antagonists) bilaterally. In sharp contrast, contractions in the 2 ASIA D subjects were reciprocal between agonists and antagonists within a limb and alternated between limbs, such that movements in these 2 subjects looked just like repetitive stepping. Finally, each of the 6 subjects had a distinct pathology of their spinal cord, nerve roots, distal trunk, or thigh; in 4 of these subjects, treatment of the pathology eliminated the involuntary movements.

CONCLUSION

The timing, distribution, and reliance upon hip angle suggest that these movement patterns reflect some elements of a central pattern generator for stepping. Emergence of these movements in persons with chronic spinal cord injury is extremely rare and appears to depend upon a combination of the more rostrally placed injury and a pathologic process leading to a further enhancement of excitability in the caudal spinal cord.

The lower limb flexion reflex in humans

The flexion or flexor reflex (FR) recorded in the lower limbs in humans (LLFR) is a widely investigated neurophysiological tool. It is a polysynaptic and multisegmental spinal response that produces a withdrawal of the stimulated limb and resembles (having several features in common) the hind-paw FR in animals. The FR, in both animals and humans, is mediated by a complex circuitry modulated at spinal and supraspinal level. At rest, the LLFR (usually obtained by stimulating the sural/tibial nerve and by recording from the biceps femoris/tibial anterior muscle) appears as a double burst composed of an early, inconstantly present component, called the RII reflex, and a late, larger and stable component, called the RIII reflex. Numerous studies have shown that the afferents mediating the RII reflex are conveyed by large-diameter, low-threshold, non-nociceptive A-beta fibers, and those mediating the RIII reflex by small-diameter, high-threshold nociceptive A-delta fibers. However, several afferents, including nociceptive and non-nociceptive fibers from skin and muscles, have been found to contribute to LLFR activation. Since the threshold of the RIII reflex has been shown to correspond to the pain threshold and the size of the reflex to be related to the level of pain perception, it has been suggested that the RIII reflex might constitute a useful tool to investigate pain processing at spinal and supraspinal level, pharmacological modulation and pathological pain conditions. As stated in EFNS guidelines, the RIII reflex is the most widely used of all the nociceptive reflexes, and appears to be the most reliable in the assessment of treatment efficacy. However, the RIII reflex use in the clinical evaluation of neuropathic pain is still limited. In addition to its nocifensive function, the LLFR seems to be linked to posture and locomotion. This may be explained by the fact that its neuronal circuitry, made up of a complex pool of interneurons, is interposed in motor control and, during movements, receives both peripheral afferents (flexion reflex afferents, FRAs) and descending commands, forming a multisensorial feedback mechanism and projecting the output to motoneurons. LLFR excitability, mediated by this complex circuitry, is finely modulated in a state- and phase-dependent manner, rather as we observe in the FR in animal models. Several studies have demonstrated that LLFR excitability may be influenced by numerous physiological conditions (menstrual cycle, stress, attention, sleep and so on) and pathological states (spinal lesions, spasticity, Wallenberg's syndrome, fibromyalgia, headaches and so on). Finally, the LLFR is modulated by several drugs and neurotransmitters. In summary, study of the LLFR in humans has proved to be an interesting functional window onto the spinal and supraspinal mechanisms of pain processing and onto the spinal neural control mechanisms operating during posture and locomotion.

Coordinated modulation of locomotor muscle synergies constructs straight-ahead and curvilinear walking in humans

We describe the muscle synergies accompanying steering of walking along curved trajectories, in order to analyze the simultaneous control of progression and balance-threatening emerging forces. For this purpose, we bilaterally recorded in ten subjects the electromyograms (EMGs) of a representative sample of leg and trunk muscles (n=16) during continuous walking along one straight and two curved trajectories at natural speed. Curvilinear locomotion involved a graded, limb-dependent modulation of amplitude and timing of activity of the muscles of the legs and trunk. The turn-related modulation of the motor pattern was highly coordinated amongst muscles and body sides. For all muscles, linear relationships were detected between the spatial and temporal features of muscle EMG activity. The largest modulation of EMG was observed in gastrocnemius medialis and lateralis muscles, which showed opposite changes in timing and amplitude during curve-walking. Moreover, amplitude and timing characteristics of muscle activities were significantly correlated with the spatial and temporal gait adaptations that are associated with curvilinear locomotion. The present results reveal that fine-modulation of the muscle synergies underlying straight-ahead locomotion is enough to generate the adequate propulsive forces to steer walking and maintain balance. These findings suggest that the turn-related command operates by modulation of the phase relationships between the tightly coupled neuronal assemblies that drive motor neuron activity during walking. This would produce the invariant templates for locomotion kinematics that are at the base of human navigation in space.

Motor Control in the Human Spinal Cord

Features of the human spinal cord motor control are described using two spinal cord injury models: (i) the spinal cord completely separated from brain motor structures by accidental injury; (ii) the spinal cord receiving reduced and altered supraspinal input due to an incomplete lesion. Systematic studies using surface electrode polyelectromyography were carried out to assess skeletal muscle reflex responses to single and repetitve stimulation in a large number of subjects. In complete spinal cord injured subjects the functional integrity of three different neuronal circuits below the lesion level is demonstrated: first, simple mono- and oligosynaptic reflex arcs and polysynaptic pathways; second, propriospinal interneuron system with their cell in the gray matter and the axons in the white matter of the spinal cord conducting activity between different spinal cord segments; and third, internuncial gray matter neurons with short axons and dense neuron contact within the spinal gray matter. All of these three systems participate continuously in the generation of spinal cord reflex output activating muscles. The integration of these systems and their relative degree of excitation and set-up produces characteristic functions of motor control. In incomplete spinal cord injured patients, the implementation of brain motor control depends on the profile of residual brain descending input and its integration with the functional neuronal circuits below the lesion. Locomotor patterns result from the establishment of a new structural relationship between brain and spinal cord. The functions of this new structural relationship are expressed as an alternative, but characteristic and consistent neurocontrol. The more we know about how the brain governs spinal cord networks, the better we can describe human motor control. On the other hand such knowledge is essential for the restoration of residual functions and for the construction of new cord circuitry to expand the functions of the injured spinal cord.

Tuning of a Basic Coordination Pattern Constructs Straight-Ahead and Curved Walking in Humans

We tested the hypothesis that common principles govern the production of the locomotor patterns for both straight-ahead and curved walking. Whole body movement recordings showed that continuous curved walking implies substantial, limb-specific changes in numerous gait descriptors. Principal component analysis (PCA) was used to uncover the spatiotemporal structure of coordination among lower limb segments. PCA revealed that the same kinematic law accounted for the coordination among lower limb segments during both straight-ahead and curved walking, in both the frontal and sagittal planes: turn-related changes in the complex behavior of the inner and outer limbs were captured in limb-specific adaptive tuning of coordination patterns. PCA was also performed on a data set including all elevation angles of limb segments and trunk, thus encompassing 13 degrees of freedom. The results showed that both straight-ahead and curved walking were low dimensional, given that 3 principal components accounted for more than 90% of data variance. Furthermore, the time course of the principal components was unchanged by curved walking, thereby indicating invariant coordination patterns among all body segments during straight-ahead and curved walking. Nevertheless, limb- and turn-dependent tuning of the coordination patterns encoded the adaptations of the limb kinematics to the actual direction of the walking body. Absence of vision had no significant effect on the intersegmental coordination during either straight-ahead or curved walking. Our findings indicate that kinematic laws, probably emerging from the interaction of spinal neural networks and mechanical oscillators, subserve the production of both straight-ahead and curved walking. During locomotion, the descending command tunes basic spinal networks so as to produce the changes in amplitude and phase relationships of the spinal output, sufficient to achieve the body turn.

Interaction between discrete and rhythmic movements: reaction time and phase of discrete movement initiation during oscillatory movements

This study investigates a task in which discrete and rhythmic movements are combined in a single-joint elbow rotation. Previous studies reported a tendency for the EMG burst associated with the discrete movement to occur around the expected burst associated with the rhythmic movement (e.g., [Exp. Brain Res. 99 (1994) 325; J. Neurol. Neurosurg. Psychiatry 40 (1977) 1129; Hum. Mov. Sci. 19 (2000) 627]). We document this interaction between discrete and rhythmic movements in different task variations and suggest a model consisting of rhythmic and discrete pattern generators that reproduces the major results. In the experiment, subjects performed single-joint elbow oscillatory movements (2 Hz). Upon a signal, they initiated a movement that consisted of a shift in the midpoint of the oscillation (MID), a shift in the amplitude of the oscillation (AMP), or a combination of both (MID + AMP). These shifting movements were performed either in a reaction time or in a self-paced fashion. The tendency for the EMG bursts associated with the discrete and rhythmic movements to synchronize was found similarly in all three tasks and instruction conditions, but the synchronization was most pronounced in the self-initiated discrete movement. Reaction time was increased for the combined task (MID + AMP), indicating higher control demands due to a combination of discrete and rhythmic components. This EMG burst synchronization was reproduced in a model based on a half-center oscillator with activation signals that produce either rhythmic or discrete activity. This activity was interpreted as torques driving a simple limb model. Summation of discrete and rhythmic activation signals of the pattern generators was sufficient to simulate the EMG burst synchronization. Further, simulation data reproduced the modulation of the reaction time as a function of the phase of the discrete movement.

Spinal cord pattern generators for locomotion

It is generally accepted that locomotion in mammals, including humans, is based on the activity of neuronal circuits within the spinal cord (the central pattern generator, CPG). Afferent information from the periphery (i.e. the limbs) influences the central pattern and, conversely, the CPG selects appropriate afferent information according to the external requirement. Both the CPG and the reflexes that mediate afferent input to the spinal cord are under the control of the brainstem. There is increasing evidence that in central motor diseases, a defective utilization of afferent input, in combination with secondary compensatory processes, is involved in typical movement disorders, such as spasticity and Parkinson's disease. Recent studies indicate a plastic behavior of the spinal neuronal circuits following a central motor lesion. This has implications for any rehabilitative therapy that should be directed to take advantage of the plasticity of the central nervous system. The significance of this research is in a better understanding of the pathophysiology underlying movement disorders and the consequences for an appropriate treatment.

Human walking along a curved path. II. Gait features and EMG patterns

We recorded basic gait features and associated patterns of leg muscle activity, occurring during continuous body progression when humans walked along a curved trajectory, in order to gain insight into the nervous mechanisms underlying the control of the asymmetric movements of the two legs. The same rhythm was propagated to both legs, in spite of inner and outer strides diverging in length (P < 0.001). There was a phase lag in limb displacement between the inner and outer leg of 7% of the total cycle duration (P = 0.0001). Swing velocity was greater for outer than inner foot (P < 0.001). The duration of the stance phase diminished and increased in the outer and inner leg (P < 0.01), respectively, and was associated with trunk leaning toward the inside of the path. Muscle activity was not dramatically altered during curved walking. The amplitude of soleus burst during stance increased in the outer (P < 0.05) and decreased in the inner leg (P < 0.05), without changes in timing. Tibialis anterior activity increased in both legs during the swing phase (P < 0.05); it was advanced on the outer and delayed on the inner side (P < 0.01; 2% of the cycle). The peroneus longus burst decreased in both legs, but more in the inner than the outer leg, and lasted longer in the inner leg at the onset of swing. Closing the eyes did not affect the gait pattern and muscle activity during turning. The command to walk along a curved path may exploit the basic mechanisms of the spinal locomotor generator, thereby limiting the computational cost of turning.

Brain activations during motor imagery of locomotor-related tasks: a PET study

Positron emission tomography (PET) was used to study the involvement of supraspinal structures in human locomotion. Six right-handed adults were scanned in four conditions while imagining locomotor-related tasks in the first person perspective: Standing (S), Initiating gait (IG), Walking (W) and Walking with obstacles (WO). When these conditions were compared to a rest (control) condition to identify the neural structures involved in the imagination of locomotor-related tasks, the results revealed a common pattern of activations, which included the dorsal premotor cortex and precuneus bilaterally, the left dorsolateral prefrontal cortex, the left inferior parietal lobule, and the right posterior cingulate cortex. Additional areas involving the pre-supplementary motor area (pre-SMA), the precentral gyrus, were activated during conditions that required the imagery of locomotor movements. Further subtractions between the different locomotor conditions were then carried out to determine the cerebral regions associated with the simulation of increasingly complex locomotor functions. These analyses revealed increases in rCBF activity in the left cuneus and left caudate when the W condition was compared to the IG condition, suggesting that the basal ganglia plays a role in locomotor movements that are automatic in nature. Finally, subtraction of the W from the WO condition yielded increases in activity in the precuneus bilaterally, the left SMA, the right parietal inferior cortex and the left parahippocampal gyrus. Altogether, the present findings suggest that higher brain centers become progressively engaged when demands of locomotor tasks require increasing cognitive and sensory information processing.

Paralysis recovery in humans and model systems

Considerable evidence now demonstrates that extensive functional and anatomical reorganization following spinal cord injury occurs in centers of the brain that have some input into spinal motor pools. This is very encouraging, given the accumulating evidence that new connections formed across spinal lesions may not be initially functionally useful. The second area of advancement in the field of paralysis recovery is in the development of effective interventions to counter axonal growth inhibition. A third area of significant progress is the development of robotic devices to quantify the performance level of motor tasks following spinal cord injury and to 'teach' the spinal cord to step and stand. Advances are being made with robotic devices for mice, rats and humans.

Lasting paraplegia caused by loss of lumbar spinal cord interneurons in rats: no direct correlation with motor neuron loss

OBJECT

The aims of this study were to investigate further the role played by lumbar spinal cord interneurons in the generation of locomotor activity and to develop a model of spinal cord injury suitable for testing neuron replacement strategies.

METHODS

Adult rats received intraspinal injections of kainic acid (KA). Locomotion was assessed weekly for 4 weeks by using the Basso, Beattie, and Bresnahan (BBB) 21-point locomotor scale, and transcranial magnetic motor evoked potentials (MMEPs) were recorded in gastrocnemius and quadriceps muscles at 1 and 4 weeks. No changes in transcranial MMEP latency were noted following KA injection, indicating that the descending motor pathways responsible for these responses, including the alpha motor neurons, were not compromised. Rats in which KA injections included much of the L-2 segment (10 animals) showed severe locomotor deficits, with a mean BBB score of 4.5 +/- 3.6 (+/- standard deviation). Rats that received lesions rostral to the L-2 segment (four animals) were able to locomote and had a mean BBB score of 14.6 +/- 2.6. Three rats that received only one injection bilaterally centered at L-2 (three animals) had a mean BBB score of 3.2 +/- 2. Histological examination revealed variable loss of motor neurons limited to the injection site. There was no correlation between motor neuron loss and BBB score.

CONCLUSIONS

Interneuron loss centered on the L-2 segment induces lasting paraplegia independent of motor neuron loss and white matter damage, supporting earlier suggestions that circuitry critical to the generator of locomotor activity (the central pattern generator) resides in this area. This injury model may prove ideal for studies of neuron replacement strategies.

Load-regulating mechanisms in gait and posture: comparative aspects

How is load sensed by receptors, and how is this sensory information used to guide locomotion? Many insights in this domain have evolved from comparative studies since it has been realized that basic principles concerning load sensing and regulation can be found in a wide variety of animals, both vertebrate and invertebrate. Feedback about load is not only derived from specific load receptors but also from other types of receptors that previously were thought to have other functions. In the central nervous system of many species, a convergence is found between specific and nonspecific load receptors. Furthermore, feedback from load receptors onto central circuits involved in the generation of rhythmic locomotor output is commonly found. During the stance phase, afferent activity from various load detectors can activate the extensor part in such circuits, thereby providing reinforcing force feedback. At the same time, the flexion is suppressed. The functional role of this arrangement is that activity in antigravity muscles is promoted while the onset of the next flexion is delayed as long as the limb is loaded. This type of reinforcing force feedback is present during gait but absent in the immoble resting animal.

The effect of transcranial magnetic stimulation on the soleus H reflex during human walking

1. The effect of transcranial magnetic stimulation (TMS) on the soleus H reflex was investigated in the stance phase of walking in seventeen human subjects. For comparison, measurements were also made during quiet standing, matched tonic plantar flexion and matched dynamic plantar flexion. 2. During walking and dynamic plantar flexion subliminal (0.95 times threshold for a motor response in the soleus muscle) TMS evoked a large short-latency facilitation (onset at conditioning-test interval: -5 to -1 ms) of the H reflex followed by a later (onset at conditioning-test interval: 3-16 ms) long-lasting inhibition. In contrast, during standing and tonic plantar flexion the short-latency facilitation was either absent or small and the late inhibition was replaced by a long-lasting facilitation. 3. When grading the intensity of TMS it was found that the short-latency facilitation had a lower threshold during walking than during standing and tonic plantar flexion. Regardless of the stimulus intensity the late facilitation was never seen during walking and dynamic plantar flexion and the late inhibition was not seen, except for one subject, during standing and tonic plantar flexion. 4. A similar difference in the threshold of the short-latency facilitation between walking and standing was not observed when the magnetic stimulation was replaced by transcranial electrical stimulation. 5. The lower threshold of the short-latency facilitation evoked by magnetic but not electrical transcranial stimulation during walking compared with standing suggests that cortical cells with direct motoneuronal connections increase their excitability in relation to human walking. The significance of the differences in the late facilitatory and inhibitory effects during the different tasks is unclear.

Evidence for a spinal central pattern generator in humans

Non-patterned electrical stimulation of the posterior structures of the lumbar spinal cord in subjects with complete, long-standing spinal cord injury, can induce patterned, locomotor-like activity. We show that epidural spinal cord stimulation can elicit step-like EMG activity and locomotor synergies in paraplegic subjects. An electrical train of stimuli applied over the second lumbar segment with a frequency of 25 to 60 Hz and an amplitude of 5-9 V was effective in inducing rhythmic, alternating stance and swing phases of the lower limbs. This finding suggests that spinal circuitry in humans has the capability of generating locomotor-like activity even when isolated from brain control, and that externally controlled sustained electrical stimulation of the spinal cord can replace the tonic drive generated by the brain.

Neural control of locomotion; The central pattern generator from cats to humans

In the last years it has become possible to regain some locomotor activity in patients suffering from an incomplete spinal cord injury (SCI) through intense training on a treadmill. The ideas behind this approach owe much to insights derived from animal studies. Many studies showed that cats with complete spinal cord transection can recover locomotor function. These observations were at the basis of the concept of the central pattern generator (CPG) located at spinal level. The evidence for such a spinal CPG in cats and primates (including man) is reviewed in part 1, with special emphasis on some very recent developments which support the view that there is a human spinal CPG for locomotion. Copyright 1997 Elsevier Science B.V.