Effects of extensor muscle afferents on the timing of locomotor activity during walking in adult rats

Spinal electrical stimulation to improve sympathetic autonomic functions needed for movement and exercise after spinal cord injury: a scoping clinical review

Spinal cord injury (SCI) results in sensory, motor and autonomic dysfunction. Obesity, cardiovascular and metabolic diseases are highly prevalent after SCI. Although inadequate voluntary activation of skeletal muscle contributes, it is absent or inadequate activation of thoracic spinal sympathetic neural circuitry and sub-optimal activation of homeostatic (cardiovascular, temperature) and metabolic support systems that truly limits exercise capacity, particularly for those with cervical SCI. Thus, when electrical spinal cord stimulation (SCS) studies aimed at improving motor functions began mentioning effects on exercise-related autonomic functions, a potential new area of clinical application appeared. To survey this new area of potential benefit, we performed a systematic scoping review of clinical SCS studies involving these spinally mediated autonomic functions. Nineteen studies were included, 8 used transcutaneous and 11 used epidural SCS. Improvements in BP at rest or in response to orthostatic challenge were investigated most systematically, whereas reports of improved temperature regulation, whole body metabolism and peak exercise performance were mainly anecdotal. Effective stimulation locations and parameters varied between studies, suggesting multiple stimulation parameters and rostrocaudal spinal locations may influence the same sympathetic function. Brainstem and spinal neural mechanisms providing excitatory drive to sympathetic neurons that activate homeostatic and metabolic tissues that provide support for movement and exercise and their integration with locomotor neural circuitry are discussed. A unifying conceptual framework for the integrated neural control of locomotor and sympathetic function is presented which may inform future research needed to take full advantage of SCS for improving these spinally mediated autonomic functions.

Behavioral testing in animal models of spinal cord injury

This review is based on a lecture presented at the Craig H. Neilsen Foundation sponsored Spinal Cord Injury Training Program at Ohio State University. We discuss the advantages and challenges of injury models in rodents and theory relation to various behavioral outcome measures. We offer strategies and advice on experimental design, behavioral testing, and on the challenges, one will encounter with animal testing. This review is designed to guide those entering the field of spinal cord injury and/or involved with in vivo animal testing.

Adaptive hindlimb split-belt treadmill walking in rats by controlling basic muscle activation patterns via phase resetting

To investigate the adaptive locomotion mechanism in animals, a split-belt treadmill has been used, which has two parallel belts to produce left–right symmetric and asymmetric environments for walking. Spinal cats walking on the treadmill have suggested the contribution of the spinal cord and associated peripheral nervous system to the adaptive locomotion. Physiological studies have shown that phase resetting of locomotor commands involving a phase shift occurs depending on the types of sensory nerves and stimulation timing, and that muscle activation patterns during walking are represented by a linear combination of a few numbers of basic temporal patterns despite the complexity of the activation patterns. Our working hypothesis was that resetting the onset timings of basic temporal patterns based on the sensory information from the leg, especially extension of hip flexors, contributes to adaptive locomotion on the split-belt treadmill. Our hypothesis was examined by conducting forward dynamic simulations using a neuromusculoskeletal model of a rat walking on a split-belt treadmill with its hindlimbs and by comparing the simulated motions with the measured motions of rats.

Depolarization and electrical stimulation enhance in vitro and in vivo sensory axon growth after spinal cord injury

Activity dependent plasticity is a key mechanism for the central nervous system (CNS) to adapt to its environment. Whether neuronal activity also influences axonal regeneration in the injured CNS, and whether electrical stimulation (ES) can activate regenerative programs in the injured CNS remains incompletely understood. Using KCl-induced depolarization, in vivo ES followed by ex-vivo neurite growth assays and ES after spinal cord lesions and cell grafting, we aimed to identify parameters important for ES-enhanced neurite growth and axonal regeneration. Using cultures of sensory neurons, neurite growth was analyzed after KCl-induced depolarization for 1-72h. Increased neurite growth was detected after short-term stimulation and after longer stimulation if a sufficient delay between stimulation and growth measurements was provided. After in vivo ES (20Hz, 2× motor threshold, 0.2ms, 1h) of the intact sciatic nerve in adult Fischer344 rats, sensory neurons showed a 2-fold increase in in vitro neurite length one week later compared to sham animals, an effect not observed one day after ES. Longer ES (7h) and repeated ES (7days, 1h each) also increased growth by 56-67% one week later, but provided no additional benefit. In vivo growth of dorsal column sensory axons into a graft of bone marrow stromal cells 4weeks after a cervical spinal cord lesion was also enhanced with a single post-injury 1h ES of the intact sciatic nerve and was also observed after repeated ES without inducing pain-like behavior. While ES did not result in sensory functional recovery, our data indicate that ES has time-dependent influences on the regenerative capacity of sensory neurons and might further enhance axonal regeneration in combinatorial approaches after SCI.

Applying a pelvic corrective force induces forced use of the paretic leg and improves paretic leg EMG activities of individuals post-stroke during treadmill walking

OBJECTIVE

To determine whether applying a mediolateral corrective force to the pelvis during treadmill walking would enhance muscle activity of the paretic leg and improve gait symmetry in individuals with post-stroke hemiparesis.

METHODS

Fifteen subjects with post-stroke hemiparesis participated in this study. A customized cable-driven robotic system based over a treadmill generated a mediolateral corrective force to the pelvis toward the paretic side during early stance phase. Three different amounts of corrective force were applied. Electromyographic (EMG) activity of the paretic leg, spatiotemporal gait parameters and pelvis lateral displacement were collected.

RESULTS

Significant increases in integrated EMG of hip abductor, medial hamstrings, soleus, rectus femoris, vastus medialis and tibialis anterior were observed when pelvic corrective force was applied, with pelvic corrective force at 9% of body weight inducing greater muscle activity than 3% or 6% of body weight. Pelvis lateral displacement was more symmetric with pelvic corrective force at 9% of body weight.

CONCLUSIONS

Applying a mediolateral pelvic corrective force toward the paretic side may enhance muscle activity of the paretic leg and improve pelvis displacement symmetry in individuals post-stroke.

SIGNIFICANCE

Forceful weight shift to the paretic side could potentially force additional use of the paretic leg and improve the walking pattern.

Distinct Functional Modules for Discrete and Rhythmic Forelimb Movements in the Mouse Motor Cortex

Movements of animals are composed of two fundamental dynamics: discrete and rhythmic movements. Although the movements with distinct dynamics are thought to be differently processed in the CNS, it is unclear how they are represented in the cerebral cortex. Here, we investigated the cortical representation of movement dynamics by developing prolonged transcranial optogenetic stimulation (pTOS) using awake, channelrhodopsin-2 transgenic mice. We found two domains that induced discrete forelimb movements in the forward and backward directions, and these sandwiched a domain that generated rhythmic forelimb movements. The forward discrete movement had an intrinsic velocity profile and the rhythmic movement had an intrinsic oscillation frequency. Each of the forward discrete and rhythmic domains possessed intracortical synaptic connections within its own domain, independently projected to the spinal cord, and weakened the neuronal activity and movement induction of the other domain. pTOS-induced movements were also classified as ethologically relevant movements. Forepaw-to-mouth movement was mapped in a part of the forward discrete domain, while locomotion-like movement was in a part of the rhythmic domain. Interestingly, photostimulation of the rhythmic domain resulted in a nonrhythmic, continuous lever-pull movement when a lever was present. The motor cortex possesses functional modules for distinct movement dynamics, and these can adapt to environmental constraints for purposeful movements. Significance statement: Animal behavior has discrete and rhythmic components, such as reaching and locomotion. It is unclear how these movements with distinct dynamics are represented in the cerebral cortex. We investigated the dynamics of movements induced by long-duration transcranial photostimulation on the dorsal cortex of awake channelrhodopsin-2 transgenic mice. We found two domains causing forward and backward discrete forelimb movements and a domain for rhythmic forelimb movements. A domain for forward discrete movement and a domain for rhythmic movement mutually weakened neuronal activity and movement size. The photostimulation of the rhythmic domain also induced nonrhythmic, lever-pull movement, when the lever was present. Thus, the motor cortex has functional modules with distinct dynamics, and each module retains flexibility for adaptation to different environments.

Control of reflex reversal in stick insect walking: effects of intersegmental signals, changes in direction, and optomotor-induced turning

In many animals, the effects of sensory feedback on motor output change during locomotion. These changes can occur as reflex reversals in which sense organs that activate muscles to counter perturbations in posture control instead reinforce movements in walking. The mechanisms underlying these changes are only partially understood. As such, it is unclear whether reflex reversals are modulated when locomotion is adapted, such as during changes in walking direction or in turning movements. We investigated these questions in the stick insect Carausius morosus, where sensory signals from the femoral chordotonal organ are known to produce resistance reflexes at rest but assistive movements during walking. We studied how intersegmental signals from neighboring legs affect the generation of reflex reversals in a semi-intact preparation that allows free leg movement during walking. We found that reflex reversal was enhanced by stepping activity of the ipsilateral neighboring rostral leg, whereas stepping of contralateral legs had no effect. Furthermore, we found that the occurrence of reflex reversals was task-specific: in the front legs of animals with five legs walking, reflex reversal was generated only during forward and not backward walking. Similarly, during optomotor-induced curved walking, reflex reversal occurred only in the middle leg on the inside of the turn and not in the contralateral leg on the outside of the turn. Thus our results show for the first time that the nervous system modulates reflexes in individual legs in the adaptation of walking to specific tasks.

A 3D analysis of fore- and hindlimb motion during locomotion: Comparison of overground and ladder walking in rats

The locomotor pattern, generated by the central pattern generator, is under the dependence of descending and peripheral pathways. The afferent feedback from peripheral receptors allows the animal to correct for disturbances that occur during walking, while supraspinal structures are important for locomotion in demanding situations such as ladder walking. Such walking, by regards to the control needed for accuracy of movements, is now widely used for description of consequences of nervous system dysfunction on motor performance. It is important to have a good knowledge of the changes in kinematic parameters according to walking conditions, since it might reflect different neural mechanisms. The aim of this work was to perform a 3D kinematic analysis of both hind- and forelimb during overground and ladder walking, to study qualitative and quantitative locomotor characteristics in different modes of locomotion. The analysis was performed on 5 rats. Movements of the right hind- and forelimb were evaluated using a 3D optical analyser, and EMG of the soleus and tibialis anterior muscles was synchronously recorded. Results indicate that kinematic and electromyographic characteristics of locomotion are dependent on the type of support. Changes were more obvious for hindlimb than for forelimb. Velocity, stride length and tibialis anterior burst duration were lower on ladder than on runway. In addition, during ladder walking, a protraction was noticed, rats bring their feet more rostral at the end of the swing phase. All these changes constitute an adaptive strategy to allow a better tactile activity with forelimbs and to avoid foot misplacement.

Vertical perturbations of human gait: organisation and adaptation of leg muscle responses

During the last several years, evidence has arisen that the neuronal control of human locomotion depends on feedback from load receptors. The aim of the present study was to determine the effects and the course of sudden and unexpected changes in body load (vertical perturbations) on leg muscle activity patterns during walking on a treadmill. Twenty-two healthy subjects walking with 25% body weight support (BWS) were repetitively and randomly loaded to 5% or unloaded to 45% BWS during left mid-stance. At the new level of BWS, the subjects performed 3-11 steps before returning to 25% BWS (base level). EMG activity of upper and lower leg muscles was recorded from both sides. The bilateral leg muscle activity pattern changed following perturbations in the lower leg muscles and the net effect of the vertical perturbations showed onset latencies with a range of 90-105 ms. Body loading enhanced while unloading diminished the magnitude of ipsilateral extensor EMG amplitude, compared to walking at base level. Contralateral leg flexor burst activity was shortened following loading and prolonged following unloading perturbation while flexor EMG amplitude was unchanged. A general decrease in EMG amplitudes occurred during the course of the experiment. This is assumed to be due to adaptation. Only the muscles directly activated by the perturbations did not significantly change EMG amplitude. This is assumed to be due to the required compensation of the perturbations by polysynaptic spinal reflexes released following the perturbations. The findings underline the importance of load receptor input for the control of locomotion.

Robot-assisted hindlimb extension increases the probability of swing initiation during treadmill walking by spinal cord contused rats

Training and evaluation of locomotion in animals with spinal cord injury will likely be improved with the development of techniques that increase stepping activity. We hypothesized that robot-assisted extension of the hindlimbs of spinal cord injured rats during stance would increase the probability that the swing phase of gait would be initiated. Thirty-three adult, Sprague-Dawley rats received a contusion injury to the mid-thoracic spinal cord. The animals' hindlimbs were pulled into extension using small robotic arms to pull at the ankle, as the rat stepped on either a reciprocating, robotic paw platform or a conventional treadmill belt. The animals demonstrated an increase in the probability of swing initiation with spontaneous recovery during the first 4 weeks following injury (p < 0.0001). The probability of swing initiation was found to be greater with the use of robot-assisted extension versus no extension force at the ankle (75+/-16.9% versus 38.9+/-16.6%, p < 0.001). Swing initiation occurred most frequently at a particular distance of hindlimb extension (50 mm caudal from the neutral position at stance), but was broadly tuned about this distance. These results indicate that a greater amount of stepping activity can be elicited by robot-assisted extension, thereby providing possible benefits to evaluation and training of gait following SCI.

Locomotor activity in spinal cord-injured persons

After a spinal cord injury (SCI) of the cat or rat, neuronal centers below the level of lesion exhibit plasticity that can be exploited by specific training paradigms. In individuals with complete or incomplete SCI, human spinal locomotor centers can be activated and modulated by locomotor training (facilitating stepping movements of the legs using body weight support on a treadmill to provide appropriate sensory cues). Individuals with incomplete SCI benefit from locomotor training such that they improve their ability to walk over ground. Load- or hip joint-related afferent input seems to be of crucial importance for both the generation of a locomotor pattern and the effectiveness of the training. However, it may be a critical combination of afferent signals that is needed to generate a locomotor pattern after severe SCI. Mobility of individuals after a SCI can be improved by taking advantage of the plasticity of the central nervous system and can be maintained with persistent locomotor activity. In the future, if regeneration approaches can successfully be applied in human SCI, even individuals with complete SCI may recover walking ability with locomotor training.

Positive force feedback in bouncing gaits?

During bouncing gaits (running, hopping, trotting), passive compliant structures (e.g. tendons, ligaments) store and release part of the stride energy. Here, active muscles must provide the required force to withstand the developing tendon strain and to compensate for the inevitable energy losses. This requires an appropriate control of muscle activation. In this study, for hopping, the potential involvement of afferent information from muscle receptors (muscle spindles, Golgi tendon organs) is investigated using a two-segment leg model with one extensor muscle. It is found that: (i) positive feedbacks of muscle-fibre length and muscle force can result in periodic bouncing; (ii) positive force feedback (F+) stabilizes bouncing patterns within a large range of stride energies (maximum hopping height of 16.3 cm, almost twofold higher than the length feedback); and (iii) when employing this reflex scheme, for moderate hopping heights (up to 8.8 cm), an overall elastic leg behaviour is predicted (hopping frequency of 1.4-3 Hz, leg stiffness of 9-27 kN m(-1)). Furthermore, F+ could stabilize running. It is suggested that, during the stance phase of bouncing tasks, the reflex-generated motor control based on feedbacks might be an efficient and reliable alternative to central motor commands.

The rat lumbosacral spinal cord adapts to robotic loading applied during stance

Load-related afferent information modifies the magnitude and timing of hindlimb muscle activity during stepping in decerebrate animals and spinal cord-injured humans and animals, suggesting that the spinal cord mediates load-related locomotor responses. In this study, we found that stepping on a treadmill by adult rats that received complete, midthoracic spinal cord transections as neonates could be altered by loading the hindlimbs using a pair of small robotic arms. The robotic arms applied a downward force to the lower shanks of the hindlimbs during the stance phase and measured the position of the lower shank during stepping. No external force was applied during the swing phase of the step. When applied bilaterally, this stance force field perturbed the hindlimb trajectories so that the ankle position was shifted downward during stance. In response to this perturbation, both the stance and step cycle durations decreased. During swing, the hindlimb initially accelerated toward the normal, unperturbed swing trajectory and then tracked the normal trajectory. Bilateral loading increased the magnitude of the medial gastrocnemius electromyographic (EMG) burst during stance and increased the amplitude of the semitendinosus and rectus femoris EMG bursts. When the force field was applied unilaterally, stance duration decreased in the loaded hindlimb, while swing duration was decreased in the contralateral hindlimb, thereby preserving interlimb coordination. These results demonstrate the feasibility of using robotic devices to mechanically modulate afferent input to the injured spinal cord during weight-supported locomotion. In addition, these results indicate that the lumbosacral spinal cord responds to load-related input applied to the lower shank during stance by modifying step timing and muscle activation patterns, while preserving normal swing kinematics and interlimb coordination.

Could enhanced reflex function contribute to improving locomotion after spinal cord repair?

Although numerous treatments have been found to improve locomotion in spinal cord injured mammals, the underlying mechanisms are very poorly understood. Some of the main possibilities are: (1) regeneration of axons across the injury site and the re-establishment of descending pathways needed to voluntarily initiate and maintain stepping in the hind legs, (2) enhanced effectiveness of undamaged neurons in preparations with incomplete transections of the cord, (3) non-specific facilitation of reflexes and intrinsic spinal networks by transmitters released from regenerated axons and/or by substances introduced by the treatment, and (4) enhanced trunk movements close to the injury site strengthening the mechanical coupling of the trunk to the hind legs via spinal reflexes. In addition, any procedure that even slightly improves stepping may be further enhanced by use-dependent modification of reflex pathways and interneuronal networks in the lumbar cord. The emphasis of this review is on the contribution of spinal reflexes to the patterning of motor activity for walking, and how enhancing reflex function may contribute to the improvement of locomotion by treatments aimed at restoring locomotion after complete transection of the spinal cord.

Hypodynamia--hypokinesia induced variations in expression of fos protein in structures related to somatosensory system in the rat

There have been many reports describing modifications of the sensory and motor cortex following various types of disuse. Hypodynamia--hypokinesia is characterized by the absence of weight-bearing and by a decrease in motor activity. We have shown a reorganization of the cortical cartography after hypodynamia--hypokinesia. In order to give an anatomical account for this cortical plasticity, we set out to determine whether cerebral and spinal structures exhibited variations of their neuronal activation. For this purpose, immunocytochemical detection of Fos protein was performed in the rat brain and spinal cord. Following stimulation of the sciatic nerve, Fos protein was detected in the primary and secondary somatosensory cortex in control rats and in rats submitted to an episode of 14 days of hypodynamia--hypokinesia. Results showed that the stimulation of the sciatic nerve induced an increase in the number of Fos-immunoreactive neurons in all these structures. Moreover, after hypodynamia--hypokinesia, the number of Fos-immunoreactive neurons was increased in the primary and secondary somatosensory cortex and in the spinal cord. These results provide evidence for a higher activation of cortical cells after hypodynamia--hypokinesia in comparison to controls. These data support the hypothesis that hypodynamia--hypokinesia contributes to the development of functional plasticity.

Proprioception from a spinocerebellar perspective

This review explores how proprioceptive sensory information is organized at spinal cord levels as it relates to a sense of body position and movement. The topic is considered in an historical context and develops a different framework that may be more in tune with current views of sensorimotor processing in other central nervous system structures. The dorsal spinocerebellar tract (DSCT) system is considered in detail as a model system that may be considered as an end point for the processing of proprioceptive sensory information in the spinal cord. An analysis of this system examines sensory processing at the lowest levels of synaptic connectivity with central neurons in the nervous system. The analysis leads to a framework for proprioception that involves a highly flexible network organization based in some way on whole limb kinematics. The functional organization underlying this framework originates with the biomechanical linkages in the limb that establish functional relationships among the limb segments. Afferent information from limb receptors is processed further through a distributed neural network in the spinal cord. The result is a global representation of hindlimb parameters rather than a muscle-by-muscle or joint-by-joint representation.

L-DOPA-induced air-stepping in the preweanling rat: electromyographic and kinematic analyses

Ontogenetic changes in intralimb coordination may result from maturation of the central pattern for locomotion, maturation of peripheral efferents, changes in afferent modulation of the centrally generated pattern, interactions with the substrate, biomechanical changes within the limb itself, or a combination of these. Electromyograms obtained from three hindlimb extensors of rats on Postnatal Days (PND) 5, 10, 15, or 20, during episodes of coordinated L-DOPA-induced air-stepping, showed that muscle activation preceded extension of the corresponding joints at all ages. The delay between the onset of extensor activity and the onset of joint extension increased during ontogeny and was greatest at PND 20. Ontogenetic changes in the relative timing of muscle activity and corresponding joint movements probably resulted from changes in biomechanical factors, changes in afferent modulation of central motor output, or both.

Modulation of cutaneous reflexes by load receptor input during human walking

To investigate the influence of load on the modulation of cutaneous reflexes, evoked by sural nerve stimulation, electromyographic activity in different leg muscles (tibialis anterior, gastrocnemius medialis (GM), biceps femoris, and soleus muscles (SO)) was recorded in healthy humans during treadmill walking with different body loads. Sural nerve stimulation was applied at two times perception threshold during different phases of the step cycle. Reflex amplitudes increased with body unloading and decreased with body loading. The reflex responses were not a simple function of the level of background activity. For example, in GM and SO, the largest reflex responses occurred during walking with body unloading, when background activity was decreased. Hence, stable ground conditions (body loading) yielded smaller reflexes. It is proposed that load receptors are involved in the regulation of cutaneous reflex responses in order to adapt the locomotor pattern to the environmental conditions.

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.

Sensory pathways and their modulation in the control of locomotion

Recent experiments have extended our understanding of how sensory information in premotor networks controlling motor output is processed during locomotion, and at what level the efficacy of specific sensory-motor pathways is determined. Phasic presynaptic inhibition of sensory transmission combined with postsynaptic alterations of excitatory and inhibitory synaptic transmission from interneurons of the premotor networks contribute to the modulation of reflex pathways and to the generation of reflex reversal. These mechanisms play an important role in adapting the operation of central networks to external demands and thus help optimize sensory-motor integration.

Decerebration by global ischemic stroke in rats

In this study we present a fast and simple technique to decerebrate rats. By injecting polyvinylsiloxane (PVS) into both common carotid arteries we occluded the circle of Willis and all cerebral arteries, thereby completely interrupting the blood supply to the cerebrum. High viscosity PVS was used so that it only entered the large arteries, and did not pass into the capillary beds. This procedure reliably resulted in an anemic decerebration, without interfering with the blood supply to the brainstem. Long-term survival was achieved by reducing intracranial pressure by the application of steroids and/or opening the skull.

Transient disturbances to one limb produce coordinated, bilateral responses during infant stepping

Transient disturbances were applied to the lower limbs of infants (3-10 mo of age) while they were supported to stepped on a treadmill. The aim was to determine how stepping infants respond to novel disturbances that would disrupt equilibrium during independent walking. Their responses were also compared with those from lower mammals and adult humans. In the first series of experiments, the motion of the limb in the swing phase was transiently stopped by the experimenter grasping the limb for a short time (0.1-1.7 s). During such disturbances, the stance phase was prolonged in the contralateral limb, and the onset of the swing phase was delayed. The degree to which the stepping was modified in the contralateral limb depended on the amount of load experienced by that limb. If the contralateral limb was bearing very little weight at the time of the disturbance, its rhythm did not change appreciably. In the second series of experiments, load was added to the infant by pushing down on the pelvis during the stance phase. This greatly prolonged the stance phase and delayed the swing phase. It did not increase the amplitude of the extensor electromyogram (EMG) of the loaded limb. In conclusion, the neural circuitry controlling stepping in the infants responds to disturbances in an organized fashion that is conducive to maintaining equilibrium and forward progression.