Plasticity of the spinal neural circuitry after injury

Metabolic costs and muscle activity patterns during robotic- and therapist-assisted treadmill walking in individuals with incomplete spinal cord injury

BACKGROUND AND PURPOSE

Robotic devices that provide passive guidance and stabilization of the legs and trunk during treadmill stepping may increase the delivery of locomotor training to subjects with neurological injury. Lower-extremity guidance also may reduce voluntary muscle activity as compared with compliant assistance provided by therapists. The purpose of this study was to investigate differences in metabolic costs and lower-limb muscle activity patterns during robotic- and therapist-assisted treadmill walking.

SUBJECTS

Twelve ambulatory subjects with motor incomplete spinal cord injury participated.

METHODS

In 2 separate protocols, metabolic and electromyographic (EMG) data were collected during standing and stepping on a treadmill with therapist and robotic assistance. During robotic-assisted walking, subjects were asked to match the kinematic trajectories of the device and maximize their effort. During therapist-assisted walking, subjects walked on the treadmill with manual assistance provided as necessary.

RESULTS

Metabolic costs and swing-phase hip flexor EMG activity were significantly lower when subjects were asked to match the robotic device trajectories than with therapist-assisted walking. These differences were reduced when subjects were asked to maximize their effort during robotic-assisted stepping, although swing-phase plantar-flexor EMG activity was increased. In addition, during standing prior to therapist- or robotic-assisted stepping, metabolic costs were higher without stabilization from the robotic device.

DISCUSSION AND CONCLUSION

Differences in metabolic costs and muscle activity patterns between therapist- and robotic-assisted standing and stepping illustrate the importance of minimizing passive guidance and stabilization provided during step training protocols.

Physical activity-mediated functional recovery after spinal cord injury: potential roles of neural stem cells

As data elucidating the complexity of spinal cord injury pathophysiology emerge, it is increasingly being recognized that successful repair will probably require a multifaceted approach that combines tactics from various biomedical disciplines, including pharmacology, cell transplantation, gene therapy and material sciences. Recently, new evidence highlighting the benefit of physical activity and rehabilitation interventions during the post-injury phase has provided novel possibilities in realizing effective repair after spinal cord injury. However, before a comprehensive therapeutic strategy that optimally utilizes the benefits of each of these disciplines can be designed, the basic mechanisms by which these various interventions act must be thoroughly explored and important synergistic and antagonistic interactions identified. In examining the mechanisms by which physical activity-based functional recovery after spinal cord injury is effected, endogenous neural stem cells, in our opinion, engender a potentially key role. Multipotent neural stem cells possess many faculties that abet recovery, including the ability to assess the local microenvironment and deliver biofactors that promote neuroplasticity and regeneration, as well as the potential to replenish damaged or eradicated cellular elements. Encouragingly, the functional recovery owing to physical activity-based therapies appears relatively robust, even when therapy is initiated in the chronic stage of spinal cord injury. In this article, we review experimental outcomes related to our hypothesis that endogenous neural stem cells mediate the functional recovery noted in spinal cord injury following physical activity-based treatments. Overall, the data advocates the incorporation of increased physical activity as a component of the multidimensional treatment of spinal cord injury and underscores the critical need to employ research-based mechanistic approaches for developing future advances in the rehabilitation of neurological injury and disorders.

Neuroplasticity after spinal cord injury and training: an emerging paradigm shift in rehabilitation and walking recovery

Physical rehabilitation after spinal cord injury has been based on the premise that the nervous system is hard-wired and irreparable. Upon this assumption, clinicians have compensated for irremediable sensorimotor deficits using braces, assistive devices, and wheelchairs to achieve upright and seated mobility. Evidence from basic science, however, demonstrates that the central nervous system after injury is malleable and can learn, and this evidence has challenged our current assumptions. The evidence is especially compelling concerning locomotion. The purpose of this perspective article is to summarize the evidence supporting an impending paradigm shift from compensation for deficits to rehabilitation as an agent for walking recovery. A physiologically based approach for the rehabilitation of walking has developed, translating evidence for activity-dependent neuroplasticity after spinal cord injury and the neurobiological control of walking. Advanced by partnerships among neuroscientists, clinicians, and researchers, critical rehabilitation concepts are emerging for activity-based therapy to improve walking recovery, with promising clinical findings.

Sprouting, regeneration and circuit formation in the injured spinal cord: factors and activity

Central nervous system (CNS) injuries are particularly traumatic, owing to the limited capabilities of the mammalian CNS for repair. Nevertheless, functional recovery is observed in patients and experimental animals, but the degree of recovery is variable. We review the crucial characteristics of mammalian spinal cord function, tract development, injury and the current experimental therapeutic approaches for repair. Regenerative or compensatory growth of neurites and the formation of new, functional circuits require spontaneous and experimental reactivation of developmental mechanisms, suppression of the growth-inhibitory properties of the adult CNS tissue and specific targeted activation of new connections by rehabilitative training.

Effect of robotic-assisted treadmill training and chronic quipazine treatment on hindlimb stepping in spinally transected rats

The purpose of this study was to determine if robotic-assisted treadmill training improved hindlimb stepping in complete spinal cord transected (ST) rats. In addition, we examined whether chronic quipazine treatment would enhance the effectiveness of robotic-assisted training. Hindlimb stepping was examined in four groups of ST rats: trained + quipazine; trained + vehicle; untrained + quipazine; and untrained + vehicle. To train the rats to step, a robotic device was used that moved the hindlimbs in a semi-fixed trajectory during treadmill stepping. The robotic device was also used to assess treadmill stepping. Quipazine or vehicle was administered to the lumbar spinal cord using an intrathecal cannula. The groups that received robotic-assisted training performed more stepping movements on the treadmill than the untrained groups 10 weeks after ST. However, no differences were found between the robotic-assisted and untrained groups 16 weeks after ST. Kinematic analyses revealed that abnormally small step cycles were performed by all of the groups of ST rats. There was no significant effect of combining robotic-assisted training and quipazine treatment on stepping recovery. These data suggest that robotic-assisted training may generate hindlimb sensory stimuli that are effective in enhancing the ability of the lumbar spinal cord to generate hindlimb stepping. However, the effectiveness of robotic-assisted training may be limited to the early stages of recovery following spinal cord transection.

Can regenerating axons recapitulate developmental guidance during recovery from spinal cord injury?

The precise wiring of the adult mammalian CNS originates during a period of stunning growth, guidance and plasticity that occurs during and shortly after development. When injured in adults, this intricate system fails to regenerate. Even when the obstacles to regeneration are cleared, growing adult CNS fibres usually remain misdirected and fail to reform functional connections. Here, we attempt to fill an important niche related to the topics of nervous system development and regeneration. We specifically contrast the difficulties faced by growing fibres within the adult context to the precise circuit-forming capabilities of developing fibres. In addition to focusing on methods to stimulate growth in the adult, we also expand on approaches to recapitulate development itself.

Glial inhibition of CNS axon regeneration

Damage to the adult CNS often leads to persistent deficits due to the inability of mature axons to regenerate after injury. Mounting evidence suggests that the glial environment of the adult CNS, which includes inhibitory molecules in CNS myelin as well as proteoglycans associated with astroglial scarring, might present a major hurdle for successful axon regeneration. Here, we evaluate the molecular basis of these inhibitory influences and their contributions to the limitation of long-distance axon repair and other types of structural plasticity. Greater insight into glial inhibition is crucial for developing therapies to promote functional recovery after neural injury.

Therapeutic interventions after spinal cord injury

Spinal cord injury (SCI) can lead to paraplegia or quadriplegia. Although there are no fully restorative treatments for SCI, various rehabilitative, cellular and molecular therapies have been tested in animal models. Many of these have reached, or are approaching, clinical trials. Here, we review these potential therapies, with an emphasis on the need for reproducible evidence of safety and efficacy. Individual therapies are unlikely to provide a panacea. Rather, we predict that combinations of strategies will lead to improvements in outcome after SCI. Basic scientific research should provide a rational basis for tailoring specific combinations of clinical therapies to different types of SCI.

Tenascin-R restricts posttraumatic remodeling of motoneuron innervation and functional recovery after spinal cord injury in adult mice

Tenascin-R (TNR) is an extracellular glycoprotein in the CNS implicated in neural development and plasticity. Its repellent properties for growing axons in a choice situation with a conducive substrate in vitro have indicated that TNR may impede regeneration in the adult mammalian CNS. Here we tested whether constitutive lack of TNR has beneficial impacts on recovery from spinal cord injury in adult mice. Using the Basso, Beattie, Bresnahan (BBB) locomotor rating scale, we found that open-field locomotion in TNR-deficient (TNR-/-) mice recovered better that in wild-type (TNR+/+) littermates after compression of the thoracic spinal cord. We also designed, validated, and applied a motion analysis approach allowing numerical assessment of motor functions. We found, in agreement with the BBB score, that functions requiring low levels of supraspinal control such as plantar stepping improved more in TNR-/- mice. This was not the case for motor tasks demanding precision such as ladder climbing. Morphological analyses revealed no evidence that improved recovery of some functions in the mutant mice were attributable to enhanced tissue sparing or axonal regrowth. Estimates of perisomatic puncta revealed reduced innervation by cholinergic and GABAergic terminals around motoneurons in intact TNR-/- compared with TNR+/+ mice. Relative to nonlesioned animals, spinal cord repair was associated with increase in GABAergic and decrease of glutamatergic puncta in TNR-/- but not in TNR+/+ mice. Our results suggest that TNR restricts functional recovery by limiting posttraumatic remodeling of synapses around motoneuronal cell bodies where TNR is normally expressed in perineuronal nets.

LOCOMOTOR CIRCUITS IN THE MAMMALIAN SPINAL CORD

Intrinsic spinal networks, known as central pattern generators (CPGs), control the timing and pattern of the muscle activity underlying locomotion in mammals. This review discusses new advances in understanding the mammalian CPGs with a focus on experiments that address the overall network structure as well as the identification of CPG neurons. I address the identification of excitatory CPG neurons and their role in rhythm generation, the organization of flexor-extensor networks, and the diverse role of commissural interneurons in coordinating left-right movements. Molecular and genetic approaches that have the potential to elucidate the function of populations of CPG interneurons are also discussed.

High Cervical Lateral Spinal Cord Injury Results in Long-Term Ipsilateral Hemidiaphragm Paralysis

Although axon regeneration is limited in the central nervous system, partial lesions of the spinal cord induce neuroplasticity processes that can lead to spontaneous functional improvement. To determine whether such compensatory mechanisms occur in the respiratory system, we analyzed the incidence of partial injury of the cervical spinal cord on diaphragm activity in adult rats. We show that a section of the lateral area of the C2 cervical spinal cord induces complete phrenic nerve inactivation and ipsilateral hemidiaphragm paralysis, whereas medial or dorsolateral sections had only a moderate effect on respiratory activity. In the case of lateral hemisection, activity of the ipsilateral phrenic nerve was partially restored after a lapse of 3 months. No spontaneous diaphragm recovery was observed, however, even after a lapse of several months in the case of hemisection or lateral section. Ipsilateral hemidiaphragm activity could however be restored after transection of the contralateral phrenic nerve, by activation of the "crossed phrenic phenomenon" (involving activation of previously latent respiratory contralateral pathways crossing the midline). These data suggest that the respiratory system develops important long-term plasticity processes at the level of phrenic motoneuron innervation. However, they do not by themselves allow substantial diaphragm recovery, underscoring the continued need for developing repair strategies. These studies also validates the use of the respiratory system as a model to evaluate the functional incidence of repair strategies not only after hemisection but also after more limited sectioning restricted to the lateral side of the cervical cord.

Degenerative and spontaneous regenerative processes after spinal cord injury

Spinal cord injury results in acute as well as progressive secondary destruction of local and distant nervous tissue through a number of degenerative mechanisms. Spinal cord injury also initiates a number of endogenous neuroprotective and regenerative responses. Understanding of these mechanisms might identify potential targets for treatments after spinal cord injury in humans. Here, we first discuss recent developments in our understanding of the immediate traumatic and subsequent secondary degeneration of local tissue and long projecting pathways in animal models. These include the inflammatory and vascular responses during the acute phase, as well as cell death, demyelination and scar formation in the subacute and chronic phases. Secondly, we discuss the spontaneous axonal regeneration of injured and plasticity of uninjured systems, and other repair-related responses in animals, including the upregulation of regeneration-associated genes in some neurons, increases in neurotrophic factors in the spinal cord and remyelination by oligodendrocyte precursors and invading Schwann cells. Lastly, we comment on the still limited understanding of the neuropathology in humans, which is largely similar to that in rodents. However, there also are potentially important differences, including the reduced glial scarring, inflammation and demyelination, the increased Schwannosis and the protracted Wallerian degeneration in humans. The validity of current rodent models for human spinal cord injury is also discussed. The emphasis of this review is on the literature from 2002 to early 2005.

Modulation of locomotor activity in complete spinal cord injury

The aim of this study was to evaluate the modulation of muscle activity during locomotor-like movements by different walking speeds in subjects with a motor complete spinal cord injury (SCI) compared to actively--and passively-walking control subjects without neurological deficit. Stepping movements on a treadmill were induced and assisted by a driven gait orthosis. Electromyographic (EMG) muscle activity of one leg (rectus and biceps femoris, tibialis anterior and gastrocnemius) was recorded and analyzed at three stepping velocities with similar body weight support in both subject groups. In SCI subjects, the EMG amplitude of biceps femoris, tibialis anterior and gastrocnemius was in general similar or weaker than in passively- and actively-stepping control subjects, but that of rectus femoris was larger. The degree of co-activation between tibialis anterior and gastrocnemius was higher in SCI than in control subjects. A significant velocity-dependent EMG modulation was present in all four-leg muscles in both subject groups. In SCI subjects, this EMG modulation was similar to that in actively stepping control subjects. It is concluded that in complete spastic SCI subjects, spinal neuronal circuits underlying locomotion can to a large extent adequately respond to a change in external drive to adapt the neuronal pattern to a new locomotion speed. The application of various speeds might enhance the effect of locomotor training in incomplete SCI subjects.

Chondroitinase ABC digestion of the perineuronal net promotes functional collateral sprouting in the cuneate nucleus after cervical spinal cord injury

Upregulation of extracellular chondroitin sulfate proteoglycans (CSPGs) after CNS injuries contributes to the impediment of functional recovery by restricting both axonal regeneration and synaptic plasticity. In the present study, the effect of degrading CSPGs with the application of the bacterial enzyme chondroitinase ABC (chABC) into the cuneate nucleus of rats partially denervated of forepaw dorsal column axons was examined. A dorsal column transection between the C6-C7 dorsal root entry zones was followed immediately by an ipsilateral brainstem injection of either chABC or a bacterial-derived control enzyme [penicillinase (P-ase)] and then subsequently (1 week later) followed with a second brainstem enzyme injection and cholera toxin B subunit (CTB) tracer injection into the ipsilateral forepaw digits and pads. After 1 additional week, the rats underwent electrophysiological receptive field mapping of the cuneate nucleus and/or anatomical evaluation. Examination of the brainstems of rats from each group revealed that CSPGs had been reduced after chABC treatment. Importantly, in the chABC-treated rats (but not in the P-ase controls), a significantly greater area of the cuneate nucleus was occupied by physiologically active CTB traced forepaw afferents that had been spared by the initial cord lesion. These results demonstrate, for the first time, a functional change directly linked to anatomical evidence of sprouting by spinal cord afferents after chABC treatment.

Spinal cord-transected mice learn to step in response to quipazine treatment and robotic training

In the present study, concurrent treatment with robotic step training and a serotonin agonist, quipazine, generated significant recovery of locomotor function in complete spinal cord-transected mice (T7-T9) that otherwise could not step. The extent of recovery achieved when these treatments were combined exceeded that obtained when either treatment was applied independently. We quantitatively analyzed the stepping characteristics of spinal mice after alternatively administering no training, manual training, robotic training, quipazine treatment, or a combination of robotic training with quipazine treatment, to examine the mechanisms by which training and quipazine treatment promote functional recovery. Using fast Fourier transform and principal components analysis, significant improvements in the step rhythm, step shape consistency, and number of weight-bearing steps were observed in robotically trained compared with manually trained or nontrained mice. In contrast, manual training had no effect on stepping performance, yielding no improvement compared with nontrained mice. Daily bolus quipazine treatment acutely improved the step shape consistency and number of steps executed by both robotically trained and nontrained mice, but these improvements did not persist after quipazine was withdrawn. At the dosage used (0.5 mg/kg body weight), quipazine appeared to facilitate, rather than directly generate, stepping, by enabling the spinal cord neural circuitry to process specific patterns of sensory information associated with weight-bearing stepping. Via this mechanism, quipazine treatment enhanced kinematically appropriate robotic training. When administered intermittently during an extended period of robotic training, quipazine revealed training-induced stepping improvements that were masked in the absence of the pharmacological treatment.

Plasticity from muscle to brain

Recognition that the entire central nervous system (CNS) is highly plastic, and that it changes continually throughout life, is a relatively new development. Until very recently, neuroscience has been dominated by the belief that the nervous system is hardwired and changes at only a few selected sites and by only a few mechanisms. Thus, it is particularly remarkable that Sir John Eccles, almost from the start of his long career nearly 80 years ago, focused repeatedly and productively on plasticity of many different kinds and in many different locations. He began with muscles, exploring their developmental plasticity and the functional effects of the level of motor unit activity and of cross-reinnervation. He moved into the spinal cord to study the effects of axotomy on motoneuron properties and the immediate and persistent functional effects of repetitive afferent stimulation. In work that combined these two areas, Eccles explored the influences of motoneurons and their muscle fibers on one another. He studied extensively simple spinal reflexes, especially stretch reflexes, exploring plasticity in these reflex pathways during development and in response to experimental manipulations of activity and innervation. In subsequent decades, Eccles focused on plasticity at central synapses in hippocampus, cerebellum, and neocortex. His endeavors extended from the plasticity associated with CNS lesions to the mechanisms responsible for the most complex and as yet mysterious products of neuronal plasticity, the substrates underlying learning and memory. At multiple levels, Eccles' work anticipated and helped shape present-day hypotheses and experiments. He provided novel observations that introduced new problems, and he produced insights that continue to be the foundation of ongoing basic and clinical research. This article reviews Eccles' experimental and theoretical contributions and their relationships to current endeavors and concepts. It emphasizes aspects of his contributions that are less well known at present and yet are directly relevant to contemporary issues.

Changes in muscle T2 relaxation properties following spinal cord injury and locomotor training

Magnetic resonance (MR) is frequently used to study structural and biochemical properties of skeletal muscle. Changes in proton transverse relaxation (T2) properties have been used to study muscle cellular damage, as well as muscle activation during exercise protocols. In this study, we implemented MR imaging to characterize the T2 relaxation properties of rat hindlimb muscles following spinal cord injury (SCI) and locomotor training. After moderate midthoracic contusion SCI, Sprague-Dawley rats were assigned to either treadmill training, cycle training or an untrained group. T2 weighted images were obtained and mean muscle T2 times were calculated in the tibialis anterior, soleus, and gastrocnemius (GAS) muscles at pre-injury as well as at 1, 2, 4, 8, and 12 weeks post-injury. Following SCI, hindlimb muscles in untrained animals showed a significant increase in muscle T2, with the most dramatic shift (+5.46 ms) observed in soleus muscle at 1 week post-SCI. Subsequently, all muscle groups showed a spontaneous recovery in muscle T2 with normalized T2 values in the GAS and tibilias anterior muscles at 4 weeks and the soleus at 12 weeks post-SCI. Both training paradigms, treadmill and cycling training, accelerated the recovery of soleus muscle T2. As a result, soleus muscle T2 recovered back to pre-injury values within 3 weeks of training in both training groups. Finally, in vitro histological assessments of rat skeletal muscles demonstrated that there was no apparent muscle injury in any of the muscles studied at 1 week post-SCI.

License to run: exercise impacts functional plasticity in the intact and injured central nervous system by using neurotrophins

Exercise has been found to impact molecular systems important for maintaining neural function and plasticity. A characteristic finding for the effects of exercise in the brain and spinal cord has been the up-regulation of brain-derived neurotrophic factor (BDNF). This review focuses on the ability of exercise to impact brain circuitry by promoting neuronal repair and enhance learning and memory by increasing neurotrophic support. A paragon for the role of activity-dependent neurotrophins in the CNS is the capacity of BDNF to facilitate synaptic function and neuronal excitability. The authors discuss the effects of exercise in the intact and injured brain and spinal cord injury and the implementation of exercise preinjury and postinjury. As the CNS displays a capacity for plasticity throughout one's lifespan, exercise may be a powerful lifestyle implementation that could be used to augment synaptic plasticity, promote behavioral rehabilitation, and counteract the deleterious effects of aging.

Rehabilitative Therapies after Spinal Cord Injury

We review some basic and highly relevant concepts in the effort to develop improved rehabilitative interventions for subjects with spinal cord injury (SCI). Interventions that are likely to contribute to improved sensorimotor function include (1) practice of the specific motor task that needs to be improved; and (2) combining the training with one or more interventions--such as pharmacological modulation of the excitability of spinal neural networks, implantation of selected cell types such as olfactory ensheathing glia (OEG), and/or modulation of the excitability of the spinal cord via epidural stimulation. Upon improvement of the neural control of the musculature following SCI, it will always be prudent to maximize the torque output from these activation patterns by assuring that muscle mass is maintained. Therefore, it seems quite feasible that considerable improvement in locomotor performance can be achieved by improved coordination of motor pools, as well as effective recovery of muscle mass, which will assist in the potential generation of normal forces among agonistic and antagonistic muscle groups.

Collateral Sprouting as a Target for Improved Function after Spinal Cord Injury

Functional recovery after spinal cord injury might be improved by enhancing the extent of innervation through stimulation of collateral sprouting, which is the growth of a new axon along the shaft of a non-injured axon. This review discusses (1) the spontaneous collateral sprouting of uninjured motor and sensory systems that has been shown after spinal cord injury in animal models, (2) experimental treatment strategies that are being developed to enhance collateral sprouting in motor systems and to reduce sensory sprouting which is associated with autonomic dysreflexia and pain, and (3) cell-surface and intracellular signaling mechanisms that are known to regulate axonal branching. The conclusion is that relatively little is known about collateral sprouting in adult mammals after spinal cord injury but that it may contribute to spontaneous functional motor recovery and causes sensory dysfunction. There is some promising data in rodents that collateral sprouting can be modulated for improved function, but the applicability to primates and relevance to human spinal cord injury remains to be determined.

Nogo-A antibody improves regeneration and locomotion of spinal cord-injured rats

Spinal cord trauma leads to loss of motor, sensory and autonomic functions below the lesion. Recovery is very restricted, due in part to neurite growth inhibitory myelin proteins, in particular Nogo-A. Two neutralizing antibodies against Nogo-A were used to study recovery and axonal regeneration after spinal cord lesions. Three months old Lewis rats were tested in sensory-motor tasks (open field locomotion, crossing of ladder rungs and narrow beams, the CatWalk(R) runway, reactions to heat and von Frey hairs). A T-shaped lesion was made at T8, and an intrathecal catheter delivered highly purified anti-Nogo-A monoclonal IgGs or unspecific IgGs for 2 weeks. A better outcome in motor behavior was obtained as early as two weeks after lesion in the animals receiving the Nogo-A antibodies. Withdrawal responses to heat and mechanical stimuli were not different between the groups. Histology showed enhanced regeneration of corticospinal axons in the anti-Nogo-A antibody groups. fMRI revealed significant cortical responses to stimulation of the hindpaw exclusively in anti-Nogo-A animals. These results demonstrate that neutralization of the neurite growth inhibitor Nogo-A by intrathecal antibodies leads to enhanced regeneration and reorganization of the injured CNS, resulting in improved recovery of compromised functions in the absence of dysfunctions.

Neurobiology of exercise

Voluntary physical activity and exercise training can favorably influence brain plasticity by facilitating neurogenerative, neuroadaptive, and neuroprotective processes. At least some of the processes are mediated by neurotrophic factors. Motor skill training and regular exercise enhance executive functions of cognition and some types of learning, including motor learning in the spinal cord. These adaptations in the central nervous system have implications for the prevention and treatment of obesity, cancer, depression, the decline in cognition associated with aging, and neurological disorders such as Parkinson's disease, Alzheimer's dementia, ischemic stroke, and head and spinal cord injury. Chronic voluntary physical activity also attenuates neural responses to stress in brain circuits responsible for regulating peripheral sympathetic activity, suggesting constraint on sympathetic responses to stress that could plausibly contribute to reductions in clinical disorders such as hypertension, heart failure, oxidative stress, and suppression of immunity. Mechanisms explaining these adaptations are not as yet known, but metabolic and neurochemical pathways among skeletal muscle, the spinal cord, and the brain offer plausible, testable mechanisms that might help explain effects of physical activity and exercise on the central nervous system.

The effects of powered ankle-foot orthoses on joint kinematics and muscle activation during walking in individuals with incomplete spinal cord injury

Background

Powered lower limb orthoses could reduce therapist labor during gait rehabilitation after neurological injury. However, it is not clear how patients respond to powered assistance during stepping. Patients might allow the orthoses to drive the movement pattern and reduce their muscle activation. The goal of this study was to test the effects of robotic assistance in subjects with incomplete spinal cord injury using pneumatically powered ankle-foot orthoses.

Methods

Five individuals with chronic incomplete spinal cord injury (ASIA C-D) participated in the study. Each subject was fitted with bilateral ankle-foot orthoses equipped with artificial pneumatic muscles to power ankle plantar flexion. Subjects walked on a treadmill with partial bodyweight support at four speeds (0.36, 0.54, 0.72 and 0.89 m/s) under three conditions: without wearing orthoses, wearing orthoses unpowered (passively), and wearing orthoses activated under pushbutton control by a physical therapist. Subjects also attempted a fourth condition wearing orthoses activated under pushbutton control by them. We measured joint angles, electromyography, and orthoses torque assistance.

Results

A therapist quickly learned to activate the artificial pneumatic muscles using the pushbuttons with the appropriate amplitude and timing. The powered orthoses provided ~50% of peak ankle torque. Ankle angle at stance push-off increased when subjects walked with powered orthoses versus when they walked with passive-orthoses (ANOVA, p < 0.05). Ankle muscle activation amplitudes were similar for powered and passive-orthoses conditions except for the soleus (~13% lower for powered condition; p < 0.05).

Two of the five subjects were able to control the orthoses themselves using the pushbuttons. The other three subjects found it too difficult to coordinate pushbutton timing. Orthoses assistance and maximum ankle angle at push-off were smaller when the subject controlled the orthoses compared to when the therapist-controlled the orthoses (p < 0.05). Muscle activation amplitudes were similar between the two powered conditions except for tibialis anterior (~31% lower for therapist-controlled; p < 0.05).

Conclusion

Mechanical assistance from powered ankle-foot orthoses improved ankle push-off kinematics without substantially reducing muscle activation during walking in subjects with incomplete spinal cord injury. These results suggest that robotic plantar flexion assistance could be used during gait rehabilitation without promoting patient passivity.

Spinal reflexes, mechanisms and concepts: From Eccles to Lundberg and beyond

This review focuses on investigations by Sir John Eccles and co-workers in Canberra, AUS in the 1950s, in which they used intracellular recordings to unravel the organization of neuronal networks in the cat spinal cord. Five classical spinal reflexes are emphasized: recurrent inhibition of motoneurons via motor axon collaterals and Renshaw cells, pathways from muscle spindles and Golgi tendon organs, presynaptic inhibition, and the flexor reflex. To set the scene for these major achievements I first provide a brief account of the understanding of the spinal cord in "reflex" and "voluntary" motor activities from the beginning of the 20th century. Next, subsequent work is reviewed on the convergence on spinal interneurons from segmental sensory afferents and descending motor pathways, much of which was performed and inspired by Anders Lundberg's group in Gothenburg, SWE. This work was the keystone for new hypotheses on the role of spinal circuits in normal motor control. Such hypotheses were later tested under more natural conditions; either by recording directly from interneurons in reduced animal preparations or by use of indirect non-invasive techniques in humans performing normal movements. Some of this latter work is also reviewed. These developments would not have been possible without the preceding work on spinal reflexes by Eccles and Lundberg. Finally, there is discussion of how Eccles' work on spinal reflexes remains central (1) as new techniques are introduced on direct recording from interneurons in behaving animals; (2) in experiments on plastic neuronal changes in relation to motor learning and neurorehabilitation; (3) in experiments on transgenic animals uncovering aspects of human pathophysiology; and (4) in evaluating the function of genetically identified classes of neurons in studies on the development of the spinal cord.

Functional plasticity following spinal cord lesions

Spinal cord injury results in marked modification and reorganization of several reflex pathways caudal to the injury. The sudden loss or disruption of descending input engenders substantial changes at the level of primary afferents, interneurons, and motoneurons thus dramatically influencing sensorimotor interactions in the spinal cord. As a general rule reflexes are initially depressed following spinal cord injury due to severe reductions in motoneuron excitability but recover and in some instances become exaggerated. It is thought that modified inhibitory connections and/or altered transmission in some of these reflex pathways after spinal injury as well as the recovery and enhancement of membrane properties in motoneurons underlie several symptoms such as spasticity and may explain some characteristics of spinal locomotion observed in spinally transected animals. Indeed, after partial or complete spinal lesions at the last thoracic vertebra cats recover locomotion when the hindlimbs are placed on a treadmill. Although some deficits in spinal locomotion are related to lesion of specific descending motor pathways, other characteristics can also be explained by changes in the excitability of reflex pathways mentioned above. Consequently it may be the case that to reestablish a stable walking pattern that modified afferent inflow to the spinal cord incurred after injury must be normalized to enable a more normal re-expression of locomotor rhythm generating networks. Indeed, recent evidence demonstrates that step training, which has extensively been shown to facilitate and ameliorate locomotor recovery in spinal animals, directly influences transmission in simple reflex pathways after complete spinal lesions.

The education and re-education of the spinal cord

In normal life, activity-dependent plasticity occurs in the spinal cord as well as in the brain. Like CNS plasticity elsewhere, this spinal cord plasticity can occur at many neuronal and synaptic sites and by a variety of mechanisms. Spinal cord plasticity is prominent in postnatal development and contributes to acquisition of standard behaviors such as locomotion and rapid withdrawal from pain. Later on in life, spinal cord plasticity contributes to acquisition and maintenance of specialized motor skills, and to compensation for the peripheral and central changes associated with aging, disease, and trauma. Mastery of even the simplest behaviors is accompanied by complex spinal and supraspinal plasticity. This complexity is necessary, to preserve the full roster of behaviors, and is also inevitable, due to the ubiquity of activity-dependent plasticity in the CNS. Careful investigation of spinal cord plasticity is essential for understanding motor skills; and, because of the relative simplicity and accessibility of the spinal cord, is a logical and convenient starting point for exploring skill acquisition. Appropriate induction and guidance of activity-dependent plasticity in the spinal cord is likely to be a key part of the realization of effective new rehabilitation methods for spinal cord injuries, cerebral palsy, and other chronic motor disorders.

Fictive locomotor patterns generated by tetraethylammonium application to the neonatal rat spinal cord in vitro

Intrinsic spinal networks generate a locomotor rhythm characterized by alternating electrical discharges from flexor and extensor motor pools. Because this process is preserved in the rat isolated spinal cord, this preparation in vitro may be a useful model to explore methods to reactivate locomotor networks damaged by spinal injury. The present electrophysiological investigation examined whether the broad spectrum potassium channel blocker tetraethylammonium could generate locomotor-like patterns. Low (50-500 microM) concentrations of tetraethylammonium induced irregular, synchronous discharges incompatible with locomotion. Higher concentrations (1-10 mM) evoked alternating discharges between flexor and extensor motor pools, plus large depolarization of motoneurons with spike broadening. The alternating discharges were superimposed on slow, shallow waves of synchronous depolarization. Rhythmic alternating patterns were suppressed by blockers of glutamate, GABA(A) and glycine receptors, disclosing a background of depolarizing bursts inhibited by antagonism of group I metabotropic glutamate receptors. Furthermore, tetraethylammonium also evoked irregular discharges on dorsal roots. Rhythmic alternating patterns elicited by tetraethylammonium on ventral roots were relatively stereotypic, had limited synergy with fictive locomotion induced by dorsal root stimuli, and were not accelerated by 4-aminopyridine. Horizontal section of the spinal cord preserved irregular ventral root discharges and dorsal root discharges, demonstrating that the action of tetraethylammonium on spinal networks was fundamentally different from that of 4-aminopyridine. These results show that a potassium channel blocker such as tetraethylammonium could activate fictive locomotion in the rat isolated spinal cord, although the pattern quality lacked certain features like frequency modulation and strong synergy with other inputs to locomotor networks.

Spinal cord injury-induced changes in breathing are not due to supraspinal plasticity in turtles (Pseudemys scripta)

After occurrence of spinal cord injury, it is not known whether the respiratory rhythm generator undergoes plasticity to compensate for respiratory insufficiency. To test this hypothesis, respiratory variables were measured in adult semiaquatic turtles using a pneumotachograph attached to a breathing chamber on a water-filled tank. Turtles breathed room air (2 h) before being challenged with two consecutive 2-h bouts of hypercapnia (2 and 6% CO2or 4 and 8% CO2). Turtles were spinalized at dorsal segments D8–D10so that only pectoral girdle movement was used for breathing. Measurements were repeated at 4 and 8 wk postinjury. For turtles breathing room air, breathing frequency, tidal volume, and ventilation were not altered by spinalization; single-breath (singlet) frequency increased sevenfold. Spinalized turtles breathing 6–8% CO2had lower ventilation due to decreased frequency and tidal volume, episodic breathing (breaths/episode) was reduced, and singlet breathing was increased sevenfold. Respiratory variables in sham-operated turtles were unaltered by surgery. Isolated brain stems from control, spinalized, and sham turtles produced similar respiratory motor output and responded the same to increased bath pH. Thus spinalized turtles compensated for pelvic girdle loss while breathing room air but were unable to compensate during hypercapnic challenges. Because isolated brain stems from control and spinalized turtles had similar respiratory motor output and chemosensitivity, breathing changes in spinalized turtles in vivo were probably not due to plasticity within the respiratory rhythm generator. Instead, caudal spinal cord damage probably disrupts spinobulbar pathways that are necessary for normal breathing.

L1 CAM expression is increased surrounding the lesion site in rats with complete spinal cord transection as neonates

L1 is a cell adhesion molecule associated with axonal outgrowth, fasciculation, and guidance during development and injury. In this study, we examined the long-term effects of spinal cord injury with and without exercise on the re-expression of L1 throughout the rat spinal cord. Spinal cords from control rats were compared to those from rats receiving complete mid-thoracic spinal cord transections at postnatal day 5, daily treadmill step training for up to 8 weeks, or both transection and step training. Three months after spinal cord transection, we observed substantially higher levels of L1 expression by both Western blot analysis and immunocytochemistry in rats with and without step training. Higher expression levels of L1 were seen in the dorsal gray matter and in the dorsal lateral funiculus both above and below the lesion site. In addition, L1 was re-expressed on the descending fibers of the corticospinal tract above the lesion. L1-labeled axons also expressed GAP-43, a protein associated with axon outgrowth and regeneration. Treadmill step training had no effect on L1 expression in either control or transected rats despite the fact that spinal transected rats displayed improved stepping patterns indicative of spinal learning. Thus, spinal cord transection at an early age induced substantial L1 expression on axons near the lesion site, but was not additionally augmented by exercise.

Animal Models for Studying Potential Training Strategies in Persons with Spinal Cord Injury

In the late 1980s, it was clearly demonstrated that adult spinal cats can be re-trained to walk after a complete spinal cord transection, using treadmill training. This has led to profound changes in the rehabilitation of persons with spinal cord injury. The use of animal models to study training-induced locomotor plasticity after spinal cord injury has expanded since this original demonstration. The goal of the present review is to summarize findings obtained with these animal models that may be of relevance to the re-training of humans with spinal cord injury. From the complete spinal cord transection models, adaptive capacity, retention of training, task-specificity, role of cutaneous inputs, effect of training with robotic devices, and spinal cord stimulation will be discussed. From the partial spinal lesion models, the effect of ventral or dorsal lesions of the cord will be presented. Finally, the effects of drugs on training will be compared between the complete and partial spinal lesions models.

Hindlimb stepping movements in complete spinal rats induced by epidural spinal cord stimulation

The locomotor ability of the spinal cord of adult rats deprived of brain control was tested by epidural spinal cord stimulation. The studies were performed on six rats that had a complete spinal cord transection (T7-T9) and epidural electrode implantations 2-3 weeks before testing was initiated. The stimulating epidural electrodes were implanted at the T12-L6 spinal segments. Epidural electrical stimulation of the dorsal surface of the spinal cord at frequencies between 1 and 50 Hz and intensities between 1 and 10 V without any pharmacological facilitation was used. Stimulation at each of the lumbar spinal cord segments elicited some rhythmic activity in the hindlimbs. However, stimulation at most segmental levels usually evoked activity in only one leg and was maintained for short periods of time (< 10s). Bilateral hindlimb locomotor activity was evoked most often with epidural stimulation at 40-50 Hz applied at the L2 segment. A necessary condition for initiation of locomotor activity was providing a specific amount (at least 5%) of body weight support. Therefore, the rat spinal cord isolated from brain control is capable of producing bilateral stepping patterns induced most readily by epidural stimulation applied at the L2 spinal segment. Furthermore, the induced stepping patterns were dependent on sensory feedback associated with weight bearing.

Performance of locomotion and foot grasping following a unilateral thoracic corticospinal tract lesion in monkeys (Macaca mulatta)

Six adult monkeys (Macaca mulatta) received a unilateral lesion of the lateral corticospinal tract (CST) in the thoracic spinal cord. Prior to surgery, the animals were trained to perform quadrupedal stepping on a treadmill, and item retrieval with the foot. Whole body kinematics and electromyogram (EMG) recordings were made prior to, and at regular intervals over a period of 12 weeks after the CST lesion. After 1 week of recovery, all monkeys were able to walk unaided quadrupedally on the treadmill. The animals, however, dragged the hindpaw ipsilateral to the lesion along the treadmill belt during the swing phase and showed a significant reorganization of the spatiotemporal pattern of hindlimb (HL) and forelimb (FL) displacements. The inability to appropriately trigger the swing phase resulted in an increase in the cycle duration and stride length of both HLs. The stance duration decreased in the ipsilateral HL, and increased in the contralateral HL and both FLs. Consequently, there was a dramatic disruption of interlimb and intralimb coupling that was reflected in the limb kinematic and EMG patterns. The CST lesion completely abolished the ability of the monkeys to retrieve items with the foot ipsilateral to the lesion and significantly disrupted the level of performance of the contralateral HL during the first 2 weeks post-lesion. Interestingly, selected HL muscles remained almost quiescent when the monkeys attempted to retrieve items, but were unsuccessful with the affected foot at 1 week post-lesion, whereas the capacity to activate the same muscles was preserved, although reduced, during stepping. Spatial and temporal parameters of gait, kinematics, and EMG patterns recorded during locomotion generally converged toward control values over time, but significant differences persisted up to 12 weeks post-lesion. Although some control was recovered over the distal foot musculature, fine foot grasping remained significantly impaired at the end of the testing period. These findings demonstrate that the CST pathway from the brain normally makes an important contribution to interlimb and intralimb coordination during basic locomotion, and to muscle activation to produce dexterous foot digit movements in the monkey. Furthermore, the present study indicates that the primate has the ability to rapidly accommodate locomotor performance, and to a lesser degree fine foot motor skills, to a reduction in supraspinal control. Identification of the neural substrates mediating the rapid recovery of motor function following injury to the primate spinal cord could provide insight into developing repair strategies to augment functional recovery from neuromotor impairments.

Impairment and recovery of postural control in rabbits with spinal cord lesions

The aim of this study was to characterize impairment and subsequent recovery of postural control after spinal cord injuries. Experiments were carried out on rabbits with three types of lesion--a dorsal (D), lateral (L), or ventral (V) hemisection (HS) at T(12) level. The animals were maintaining equilibrium on a platform periodically tilted in the frontal plane. We assessed the postural limb/trunk configuration from video recordings and postural reflexes in the hindquarters from kinematical and electromyographic (EMG) recordings. We found that for a few days after DHS or LHS, the animals were not able to maintain the dorsal-side-up position of their hindquarters. This ability was then gradually restored, and the dynamic postural reflexes reached the prelesion value within 2-3 wk. By contrast, a VHS almost completely abolished postural reflexes, and they did not recover for > or =7 wk. The DHS, LHS, and VHS caused immediate and slowly compensated changes in the postural limb/trunk configuration as well as gradually developing changes. After DHS, both hind limbs were placed in an abnormal rostral and medial position. After LHS, the limb on the undamaged side was turned inward and occurred at the abnormal medial position; LHS also caused a gradually developing twisting of the caudal trunk. VHS caused gradually developing extension of the ankle and knee joints. These findings show that ventral spinal pathways are of crucial importance for postural control. When a part of these pathways is spared, postural reflexes can be restored rapidly, but not the postural limb/trunk configuration. Spinal and supraspinal mechanisms responsible for postural deficits and their compensation are discussed.

Modulation of hypoglossal motoneuron excitability by intracellular signal transduction cascades

Motoneuronal excitability is highly modulated by various inputs; however, comparatively little is known about postsynaptic signal transduction cascades that affect motoneuron excitability. In this review, we discuss the role of intracellular signaling cascades in the modulation of respiratory motoneuronal excitability. In particular, protein kinases and phosphatases dynamically and constitutively modulate respiratory-modulated inputs to XII motoneurons: (i) activation of protein kinase A (PKA) potentiates both excitatory and inhibitory drive currents; (ii) protein kinase G (PKG) depresses excitatory currents, and (iii) inhibition of protein phosphatases potentiates excitatory drive currents. We also describe a novel form of persistent plasticity (in vitro long-term facilitation; ivLTF) of motoneuronal output. ivLTF is induced by episodic activation of 5-HT(2) or alpha(1)-adrenoreceptors and is manifested as an increase in the amplitude of XII nerve output due to an increase in alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-mediated motoneuronal drive currents. Blockade of Group 1 metabotropic glutamate receptors or protein kinase C (PKC) prevents the induction of ivLTF.

Exercise restores levels of neurotrophins and synaptic plasticity following spinal cord injury

We have conducted studies to determine the potential of exercise to benefit the injured spinal cord using neurotrophins. Adult rats were randomly assigned to one of three groups: (1) intact control (Con); (2) sedentary, hemisected at a mid-thoracic level (Sed-Hx), or (3) exercised, hemisected (Ex-Hx). One week after surgery, the Ex-Hx rats were exposed to voluntary running wheels for 3, 7, or 28 days. BDNF mRNA levels on the lesioned side of the spinal cord lumbar region of Sed-Hx rats were approximately 80% of Con values at all time points and BDNF protein levels were approximately 40% of Con at 28 days. Exercise compensated for the reductions in BDNF after hemisection, such that BDNF mRNA levels in the Ex-Hx rats were similar to Con after 3 days and higher than Con after 7 (17%) and 28 (27%) days of exercise. After 28 days of exercise, BDNF protein levels were 33% higher in Ex-Hx than Con rats and were highly correlated (r=0.86) to running distance. The levels of the downstream effectors for the action of BDNF on synaptic plasticity synapsin I and CREB were lower in Sed-Hx than Con rats at all time points. Synapsin I mRNA and protein levels were higher in Ex-Hx rats than Sed-Hx rats and similar to Con rats at 28 days. CREB mRNA values were higher in Ex-Hx than Sed-Hx rats at all time points. Hemisection had no significant effects on the levels of NT-3 mRNA or protein; however, voluntary exercise resulted in an increase in NT-3 mRNA levels after 28 days (145%). These results are consistent with the concept that synaptic pathways under the regulatory role of BDNF induced by exercise can play a role in facilitating recovery of locomotion following spinal cord injury.

L1 CAM expression in the superficial dorsal horn is derived from the dorsal root ganglion

The cell adhesion molecule L1 is highly expressed on embryonic axons and may play a role in axonal outgrowth and fasciculation. Generally only low levels of L1 are found in adult spinal cord except for intense labeling in Lissauer's tract, in laminae I-II, and on dorsolateral funicular axons. In this study we determine the source of L1 immunoreactivity in the dorsal spinal cord, the presence of L1 expression on sprouting axons, and the effect of exercise on L1 expression. We determined the source of L1 immunoreactivity in the superficial dorsal horn by performing acute unilateral rhizotomies (T12-L4) in adult rats. This resulted in a marked decrease in L1 expression in Lissauer's tract and laminae I-II on the deafferented side. The peptidergic and nonpeptidergic small-diameter primary afferent markers, calcitonin gene-related peptide (CGRP) and the lectin IB4 respectively, closely correlated with L1 expression and also decreased dramatically after rhizotomy. Considering its developmental role, we asked whether L1 was expressed on sprouting axons following chronic rhizotomy. L1 and CGRP, but not IB4, were detected on sprouting axons. Lastly, we investigated the effect of exercise on L1 expression by giving animals with chronic rhizotomies free access to an exercise wheel. After extensive exercise, L1, CGRP, and IB4 expression levels were unchanged compared with those of sedentary chronic animals. Combined, these data demonstrate that the dorsal root ganglia is a major source of L1-positive axons in the superficial dorsal horn, that both L1 and CGRP identify sprouting axons following rhizotomy, and that exercise does not upregulate L1 expression.

Exercise leads to faster postural reflexes, improved balance and mobility, and fewer falls in older persons with chronic stroke

OBJECTIVES

To determine the effect of two different community-based group exercise programs on functional balance, mobility, postural reflexes, and falls in older adults with chronic stroke.

DESIGN

A randomized, clinical trial.

SETTING

Community center.

PARTICIPANTS

Sixty-one community-dwelling older adults with chronic stroke.

INTERVENTION

Participants were randomly assigned to an agility (n=30) or stretching/weight-shifting (n=31) exercise group. Both groups exercised three times a week for 10 weeks.

MEASUREMENTS

Participants were assessed before, immediately after, and 1 month after the intervention for Berg Balance, Timed Up and Go, step reaction time, Activities-specific Balance Confidence, and Nottingham Health Profile. Testing of standing postural reflexes and induced falls evoked by a translating platform was also performed. In addition, falls in the community were tracked for 1 year from the start of the interventions.

RESULTS

Although exercise led to improvements in all clinical outcome measures for both groups, the agility group demonstrated greater improvement in step reaction time and paretic rectus femoris postural reflex onset latency than the stretching/weight-shifting group. In addition, the agility group experienced fewer induced falls on the platform.

CONCLUSION

Group exercise programs that include agility or stretching/weight shifting exercises improve postural reflexes, functional balance, and mobility and may lead to a reduction of falls in older adults with stroke.

Cell death and axon regeneration of Purkinje cells after axotomy: challenges of classical hypotheses of axon regeneration

Although adult mammalian neurons are able to regenerate their axons in the peripheral nervous system under certain conditions, they are not able to do it in the central nervous system. The environment surrounding the severed axons appears to be a key factor for axon regeneration. Many studies aiming to enhance axon regeneration in the CNS of adult mammals have successfully manipulated this environment by adding growth permissive molecules and/or neutralizing growth inhibitory molecules. In both cases, the number of axons able to regenerate was low and the different neuronal populations were not equal in their regenerative response, suggesting that manipulation of the environment is not always sufficient. This is particularly well illustrated in the cerebellar system, in which axotomized inferior olivary neurons regenerate when confronted with a permissive environment, whereas mature Purkinje cells do not. The intrinsic ability of a neuron to regenerate its axon is generally correlated with the intensity of its reaction to axotomy (expression of molecules, probability to die). Furthermore, molecules such as GAP-43 (growth-associated molecule) and c-Jun are involved in both axon regeneration and cell death suggesting that these two processes are linked. Surprisingly, Purkinje cells lose their capacity to regenerate their axon (even in the absence of myelin) during development before losing their capacity to react to an axotomy by cell death. These results emphasize the different reactions to axotomy between neuron types and underline that in Purkinje cells, the two cell decisions (axon regeneration and cell death) are differently regulated and therefore not part of the same signaling pathway.

Kinematic and EMG determinants in quadrupedal locomotion of a non-human primate (Rhesus)

We hypothesized that the activation patterns of flexor and extensor muscles and the resulting kinematics of the forelimbs and hindlimbs during locomotion in the Rhesus would have unique characteristics relative to other quadrupedal mammals. Adaptations of limb movements and in motor pool recruitment patterns in accommodating a range of treadmill speeds similar to other terrestrial animals in both the hindlimb and forelimb were observed. Flexor and extensor motor neurons from motor pools in the lumbar segments, however, were more highly coordinated than in the cervical segments. Unlike the lateral sequence characterizing subprimate quadrupedal locomotion, non-human primates use diagonal coordination between the hindlimbs and forelimbs, similar to that observed in humans between the legs and arms. Although there was a high level of coordination between hind- and forelimb locomotion kinematics, limb-specific neural control strategies were evident in the intersegmental coordination patterns and limb endpoint trajectories. Based on limb kinematics and muscle recruitment patterns, it appears that the hindlimbs, and notably the distal extremities, contribute more to body propulsion than the forelimbs. Furthermore, we found adaptive changes in the recruitment patterns of distal muscles in the hind- and forelimb with increased treadmill speed that likely correlate with the anatomical and functional evolution of hand and foot digits in monkeys. Changes in the properties of both the spinal and supraspinal circuitry related to stepping, probably account for the peculiarities in the kinematic and EMG properties during non-human primate locomotion. We suggest that such adaptive changes may have facilitated evolution toward bipedal locomotion.

Powered lower limb orthoses for gait rehabilitation

Bodyweight supported treadmill training has become a prominent gait rehabilitation method in leading rehabilitation centers. This type of locomotor training has many functional benefits but the labor costs are considerable. To reduce therapist effort, several groups have developed large robotic devices for assisting treadmill stepping. A complementary approach that has not been adequately explored is to use powered lower limb orthoses for locomotor training. Recent advances in robotic technology have made lightweight powered orthoses feasible and practical. An advantage to using powered orthoses as rehabilitation aids is they allow practice starting, turning, stopping, and avoiding obstacles during overground walking.

Deafferentation and neurotrophin-mediated intraspinal sprouting: a central role for the p75 neurotrophin receptor

Axonal plasticity in the adult spinal cord is governed by intrinsic neuronal growth potential and by extracellular cues. The p75 receptor (p75(NTR)) binds growth-promoting neurotrophins (NTs) as well as the common receptor for growth-inhibiting myelin-derived proteins (the Nogo receptor) and so is well situated to gauge the balance of positive and negative influences on axonal plasticity. Using transgenic mice lacking the extracellular NT-binding domain of p75(NTR) (p75-/- mice), we have examined the influence of p75(NTR) on changes in the density of primary afferent (calcitonin gene-related peptide-expressing) and descending monoaminergic (serotonin- and tyrosine hydroxylase-expressing) projections to the dorsal horn after dorsal rhizotomy, with and without concomitant application of exogenous nerve growth factor and NT-3. We found that, in intact p75-/- mice, the axon density of all populations was equal to or less than that in wild-type mice but that rhizotomy-induced intraspinal sprouting was significantly augmented. Monoaminergic axon sprouting was enhanced in both nerve growth factor- and NT-3-treated p75-/- mice compared with similarly treated wild-type mice. Primary afferent sprouting was particularly robust in NT-3-treated p75-/- mice. These in vivo results illustrate the interactions of p75(NTR) with NTs, with their respective tropomyosin-related kinase receptors and with inhibitory myelin-derived molecules. Our findings illustrate the pivotal role of p75(NTR) in spinal axonal plasticity and identify it as a potential therapeutic target for spinal cord injury.