Effects of bilateral lesions of the dorsolateral funiculi and dorsal columns at the level of the low thoracic spinal cord on the control of locomotion in the adult cat. I. Treadmill walking

Uncovering and leveraging the return of voluntary motor programs after paralysis using a bi-cortical neuroprosthesis

Rehabilitative and neuroprosthetic approaches after spinal cord injury (SCI) aim to reestablish voluntary control of movement. Promoting recovery requires a mechanistic understanding of the return of volition over action, but the relationship between re-emerging cortical commands and the return of locomotion is not well established. We introduced a neuroprosthesis delivering targeted bi-cortical stimulation in a clinically relevant contusive SCI model. In healthy and SCI cats, we controlled hindlimb locomotor output by tuning stimulation timing, duration, amplitude, and site. In intact cats, we unveiled a large repertoire of motor programs. After SCI, the evoked hindlimb lifts were highly stereotyped, yet effective in modulating gait and alleviating bilateral foot drag. Results suggest that the neural substrate underpinning motor recovery had traded-off selectivity for efficacy. Longitudinal tests revealed that the return of locomotion after SCI was correlated with recovery of the descending drive, which advocates for rehabilitation interventions directed at the cortical target.

A secondary motor area contributing to interlimb coordination during visually guided locomotion in the cat

We investigated the contribution of cytoarchitectonic cortical area 4δc, in the caudal bank of the cruciate sulcus of the cat, to the control of visually guided locomotion. To do so, we recorded the activity of 114 neurons in 4δc while cats walked on a treadmill and stepped over an obstacle that advanced toward them. A total of 84/114 (74%) cells were task-related and 68/84 (81%) of these cells showed significant modulation of their discharge frequency when the contralateral limbs were the first to step over the obstacle. These latter cells included a substantial proportion (27/68 40%) that discharged between the passage of the contralateral forelimb and the contralateral hindlimb over the obstacle, suggesting a contribution of this area to interlimb coordination. We further compared the discharge in area 4δc with the activity patterns of cells in the rostral division of the same cytoarchitectonic area (4δr), which has been suggested to be a separate functional region. Despite some differences in the patterns of activity in the 2 subdivisions, we suggest that activity in each is compatible with a contribution to interlimb coordination and that they should be considered as a single functional area that contributes to both forelimb-forelimb and forelimb-hindlimb coordination.

Modulation of the gait pattern during split-belt locomotion after lateral spinal cord hemisection in adult cats

Most previous studies investigated the recovery of locomotion in animals and people with incomplete spinal cord injury (SCI) during relatively simple tasks (e.g., walking in a straight line on a horizontal surface or a treadmill). We know less about the recovery of locomotion after incomplete SCI in left-right asymmetric conditions, such as turning or stepping along circular trajectories. To investigate this, we collected kinematic and electromyography data during split-belt locomotion at different left-right speed differences before and after a right thoracic lateral spinal cord hemisection in nine adult cats. After hemisection, although cats still performed split-belt locomotion, we observed several changes in the gait pattern compared with the intact state at early (1-2 wk) and late (7-8 wk) time points. Cats with larger lesions showed new coordination patterns between the fore- and hindlimbs, with the forelimbs taking more steps. Despite this change in fore-hind coordination, cats maintained consistent phasing between the fore- and hindlimbs. Adjustments in cycle and phase (stance and swing) durations between the slow and fast sides allowed animals to maintain 1:1 left-right coordination. Periods of triple support involving the right (ipsilesional) hindlimb decreased in favor of quad support and triple support involving the other limbs. Step and stride lengths decreased with concurrent changes in the right fore- and hindlimbs, possibly to avoid interference. The above adjustments in the gait pattern allowed cats to retain the ability to locomote in asymmetric conditions after incomplete SCI. We discuss potential plastic neuromechanical mechanisms involved in locomotor recovery in these conditions.NEW & NOTEWORTHY Everyday locomotion often involves left-right asymmetries, when turning, walking along circular paths, stepping on uneven terrains, etc. To show how incomplete spinal cord injury affects locomotor control in asymmetric conditions, we collected data before and after a thoracic lateral spinal hemisection on a split-belt treadmill with one side stepping faster than the other. We show that adjustments in kinematics and muscle activity allowed cats to retain the ability to perform asymmetric locomotion after hemisection.

Corticospinal Excitability Quantification During a Visually-Guided Precision Walking Task in Humans: Potential for Neurorehabilitation

The corticospinal tract has been shown to be involved in normal walking in humans. However, its contribution during more challenging locomotor tasks is still unclear. As the corticospinal tract can be a potential target to promote gait recovery after neurological injury, it is of primary importance to quantify its use during human walking. The aims of the current study were to: (1) quantify the effects of precision walking on corticospinal excitability as compared to normal walking; (2) assess if corticospinal modulation is related to task difficulty or participants' performance. Sixteen healthy participants walked on a treadmill during 2 tasks: regular walking (simple task) and stepping onto virtual targets (precision task). Virtual targets appeared randomly at 3 different step lengths: preferred, and ±20%. To assess corticospinal excitability, 25 motor evoked potentials (MEPs) were recorded from the tibialis anterior muscle in each task during walking. Performance for each participant (global success score; % of target hit) and task difficulty related to step length adjustments (success score for each step length) were also calculated. MEP size was larger during the precision task in all participants (mean increase of 93% ± 72%; P < .05) compared to the simple task. There was a correlation between MEP facilitation and individual performance (r = -.64; P < .05), but no difference in MEP size associated with task difficulty (P > .05). In conclusion, corticospinal excitability exhibits a large increase during the precision task. This effect needs to be confirmed in neurological populations to potentially provide a simple and non-invasive approach to increase corticospinal drive during gait rehabilitation.

Control of fore- and hindlimb movements and their coordination during quadrupedal locomotion across speeds in adult spinal cats

Coordinating the four limbs is critical for terrestrial mammalian locomotion. Thoracic spinal transection abolishes neural communication between the brain and spinal networks controlling hindlimb/leg movements. Several studies have shown that animal models of spinal transection (spinalization), such as mice, rats, cats and dogs recover hindlimb locomotion with the forelimbs stationary or suspended. However, we know less on the ability to generate quadrupedal locomotion after spinal transection. We collected kinematic and electromyography data in four adult cats during quadrupedal locomotion at five treadmill speeds before (intact cats) and after low-thoracic spinal transection (spinal cats). We show that adult spinal cats performed quadrupedal treadmill locomotion and modulated their speed from 0.4 m/s to 0.8 m/s but required perineal stimulation. During quadrupedal locomotion, several compensatory strategies occurred, such as postural adjustments of the head and neck and the appearance of new coordination patterns between the fore- and hindlimbs, where the hindlimbs took more steps than the forelimbs. We also observed temporal changes, such as shorter forelimb cycle/swing durations and shorter hindlimb cycle/stance durations in the spinal state. Forelimb double support periods occupied a greater proportion of the cycle in the spinal state and hindlimb stride length was shorter. Coordination between the fore- and hindlimbs was weakened and more variable in the spinal state. Changes in muscle activity reflected spatiotemporal changes in the locomotor pattern. Despite important changes in the pattern, our results indicate that biomechanical properties of the musculoskeletal system play an important role in quadrupedal locomotion and offset some of the loss in neural communication between networks controlling the fore- and hindlimbs following spinal transection.

Quantitative electrophysiological assessments as predictive markers of lower limb motor recovery after spinal cord injury: a pilot study with an adaptive trial design

Study design

Observational, cohort study.

Objectives

(1) Determine the feasibility and relevance of assessing corticospinal, sensory, and spinal pathways early after traumatic spinal cord injury (SCI) in a rehabilitation setting. (2) Validate whether electrophysiological and magnetic resonance imaging (MRI) measures taken early after SCI could identify preserved neural pathways, which could then guide therapy.

Setting

Intensive functional rehabilitation hospital (IFR).

Methods

Five individuals with traumatic SCI and eight controls were recruited. The lower extremity motor score (LEMS), electrical perceptual threshold (EPT) at the S2 dermatome, soleus (SOL) H-reflex, and motor evoked potentials (MEPs) in the tibialis anterior (TA) muscle were assessed during the stay in IFR and in the chronic stage (>6 months post-SCI). Control participants were only assessed once. Feasibility criteria included the absence of adverse events, adequate experimental session duration, and complete dataset gathering. The relationship between electrophysiological data collected in IFR and LEMS in the chronic phase was studied. The admission MRI was used to calculate the maximal spinal cord compression (MSCC).

Results

No adverse events occurred, but a complete dataset could not be collected for all subjects due to set-up configuration limitations and time constraints. EPT measured at IFR correlated with LEMS in the chronic phases (r = −0.67), whereas SOL H/M ratio, H latency, MEPs and MSCC did not.

Conclusions

Adjustments are necessary to implement electrophysiological assessments in an IFR setting. Combining MRI and electrophysiological measures may lead to better assessment of neuronal deficits early after SCI.

Effect of spinal cord injury on neural encoding of spontaneous postural perturbations in the hindlimb sensorimotor cortex

Supraspinal signals play a significant role in compensatory responses to postural perturbations. Although the cortex is not necessary for basic postural tasks in intact animals, its role in responding to unexpected postural perturbations after spinal cord injury (SCI) has not been studied. To better understand how SCI impacts cortical encoding of postural perturbations, the activity of single neurons in the hindlimb sensorimotor cortex (HLSMC) was recorded in the rat during unexpected tilts before and after a complete midthoracic spinal transection. In a subset of animals, limb ground reaction forces were also collected. HLSMC activity was strongly modulated in response to different tilt profiles. As the velocity of the tilt increased, more information was conveyed by the HLSMC neurons about the perturbation due to increases in both the number of recruited neurons and the magnitude of their responses. SCI led to attenuated and delayed hindlimb ground reaction forces. However, HLSMC neurons remained responsive to tilts after injury but with increased latencies and decreased tuning to slower tilts. Information conveyed by cortical neurons about the tilts was therefore reduced after SCI, requiring more cells to convey the same amount of information as before the transection. Given that reorganization of the hindlimb sensorimotor cortex in response to therapy after complete midthoracic SCI is necessary for behavioral recovery, this sustained encoding of information after SCI could be a substrate for the reorganization that uses sensory information from above the lesion to control trunk muscles that permit weight-supported stepping and postural control.NEW & NOTEWORTHY The role of cortical circuits in the encoding of posture and balance is of interest for developing therapies for spinal cord injury. This work demonstrated that unexpected postural perturbations are encoded in the hindlimb sensorimotor cortex even in the absence of hindlimb sensory feedback. In fact, the hindlimb sensorimotor cortex continues to encode for postural perturbations after complete spinal transection.

Microstimulation of the Premotor Cortex of the Cat Produces Phase-Dependent Changes in Locomotor Activity

To determine the functional organization of premotor areas in the cat pericruciate cortex we applied intracortical microstimulation (ICMS) within multiple cytoarchitectonically identified subregions of areas 4 and 6 in the awake cat, both at rest and during treadmill walking. ICMS in most premotor areas evoked clear twitch responses in the limbs and/or head at rest. During locomotion, these same areas produced phase-dependent modifications of muscle activity. ICMS in the primary motor cortex (area 4γ) produced large phase-dependent responses, mostly restricted to the contralateral forelimb or hindlimb. Stimulation in premotor areas also produced phase-dependent responses that, in some cases, were as large as those evoked from area 4γ. However, responses from premotor areas had more widespread effects on multiple limbs, including the ipsilateral limbs, than did stimulation in 4γ. During locomotion, responses in both forelimb and hindlimb muscles were evoked from cytoarchitectonic areas 4γ, 4δ, 6aα, and 6aγ. However, the prevalence of effects in a given limb varied from one area to another. The results suggest that premotor areas may contribute to the production, modification, and coordination of activity in the limbs during locomotion and may be particularly pertinent during modifications of gait.

Premotor Cortex Provides a Substrate for the Temporal Transformation of Information During the Planning of Gait Modifications

We tested the hypothesis that the premotor cortex (PMC) in the cat contributes to the planning and execution of visually guided gait modifications. We analyzed single unit activity from 136 cells localized within layer V of cytoarchitectonic areas 6iffu and that part of 4δ within the ventral bank of the cruciate sulcus while cats walked on a treadmill and stepped over an obstacle that advanced toward them. We found a rich variety of discharge patterns, ranging from limb-independent cells that discharged several steps in front of the obstacle to step-related cells that discharged either during steps over the obstacle or in the steps leading up to that step. We propose that this population of task-related cells within this region of the PMC contributes to the temporal evolution of a planning process that transforms global information of the presence of an obstacle into the precise spatio-temporal limb adjustment required to negotiate that obstacle.

Recovery of locomotion in cats after severe contusion of the low thoracic spinal cord

Large bilateral contusions of the T10 thoracic spinal cord were performed in 16 adult cats using a calibrated impactor. EMG and video recordings allowed weekly assessments of key locomotor parameters during treadmill training for 5 wk. Thirty-five days postcontusion, several hindlimb locomotor parameters were very similar to the prelesion ones despite some long-term deficits such as paw drag and disrupted fore-hindlimb coupling. Nine out of ten tested cats could step over obstacles placed on the treadmill. Acute electrophysiological experiments showed viable connectivity between segments rostral and caudal to the contusion. At the fifth postcontusion week, a complete spinalization was performed at T13 in 10 cats and all expressed remarkable bilateral hindlimb locomotion within 24-72 h. From our histological evaluation, we concluded that only a small percentage (~10%) of spinal cord pathways was necessary to initiate and maintain a voluntary quadrupedal locomotor pattern on a treadmill and even to negotiate obstacles. Our findings suggest that hindlimb stepping largely resulted from the activity of spinal locomotor circuits, which gradually recovered autonomy week after week. Our histological and electrophysiological evidence indicated that the persistence of specific deficits or else the maintenance of specific functions was related to the integrity of specific supraspinal and propriospinal pathways. The conclusion is that the recovery of locomotion after large spinal contusions depends on a homeostatic recalibration of a tripartite control system involving interactions between spinal circuits (central pattern generator), supraspinal influences, and sensory feedback activated through locomotor training.NEW & NOTEWORTHY The recovery of quadrupedal treadmill locomotion after a large bilateral contusion at the low thoracic T10 spinal level and the ability to negotiate obstacles were studied for 5 wk in 16 cats. Ten cats were further completely spinalized at T13 and were found to walk with the hindlimbs within 24-72 h. We conclude that the extent of locomotor recovery after large spinal contusions hinges both on remnant supraspinal pathways and on a spinal pattern generator.

Effect of cervicolumbar coupling on spinal reflexes during cycling after incomplete spinal cord injury

Spinal networks in the cervical and lumbar cord are actively coupled during locomotion to coordinate arm and leg activity. The goals of this project were to investigate the intersegmental cervicolumbar connectivity during cycling after incomplete spinal cord injury (iSCI) and to assess the effect of rehabilitation training on improving reflex modulation mediated by cervicolumbar pathways. Two studies were conducted. In the first, 22 neurologically intact (NI) people and 10 people with chronic iSCI were recruited. The change in H-reflex amplitude in flexor carpi radialis (FCR) during leg cycling and H-reflex amplitude in soleus (SOL) during arm cycling were investigated. In the second study, two groups of participants with chronic iSCI underwent 12 wk of cycling training: one performed combined arm and leg cycling (A&L) and the other legs only cycling (Leg). The effect of training paradigm on the amplitude of the SOL H-reflex was assessed. Significant reduction in the amplitude of both FCR and SOL H-reflexes during dynamic cycling of the opposite limbs was found in NI participants but not in participants with iSCI. Nonetheless, there was a significant reduction in the SOL H-reflex during dynamic arm cycling in iSCI participants after training. Substantial improvements in SOL H-reflex properties were found in the A&L group after training. The results demonstrate that cervicolumbar modulation during rhythmic movements is disrupted in people with chronic iSCI; however, this modulation is restored after cycling training. Furthermore, involvement of the arms simultaneously with the legs during training may better regulate the leg spinal reflexes.

NEW & NOTEWORTHY This work systematically demonstrates the disruptive effect of incomplete spinal cord injury on cervicolumbar coupling during rhythmic locomotor movements. It also shows that the impaired cervicolumbar coupling could be significantly restored after cycling training. Actively engaging the arms in rehabilitation paradigms for the improvement of walking substantially regulates the excitability of the lumbar spinal networks. The resulting regulation may be better than that obtained by interventions that focus on training of the legs only.

Spinal control of muscle synergies for adult mammalian locomotion

KEY POINTS

The control of locomotion is thought to be generated by activating groups of muscles that perform similar actions, which are termed muscle synergies. Here, we investigated if muscle synergies are controlled at the level of the spinal cord. We did this by comparing muscle activity in the legs of cats during stepping on a treadmill before and after a complete spinal transection that abolishes commands from the brain. We show that muscle synergies were maintained following spinal transection, validating the concept that muscle synergies for locomotion are primarily controlled by circuits of neurons within the spinal cord.

ABSTRACT

Locomotion is thought to involve the sequential activation of functional modules or muscle synergies. Here, we tested the hypothesis that muscle synergies for locomotion are organized within the spinal cord. We recorded bursts of muscle activity in the same cats (n = 7) before and after spinal transection during tied-belt locomotion at three speeds and split-belt locomotion at three left-right speed differences. We identified seven muscles synergies before (intact state) and after (spinal state) spinal transection. The muscles comprising the different synergies were the same in the intact and spinal states as well as at different speeds or left-right speed differences. However, there were some significant shifts in the onsets and offsets of certain synergies as a function of state, speed and left-right speed differences. The most notable difference between the intact and spinal states was a change in the timing between the knee flexor and hip flexor muscle synergies. In the intact state, the knee flexor synergy preceded the hip flexor synergy, whereas in the spinal state both synergies occurred concurrently. Afferent inputs also appear important for the expression of some muscle synergies, specifically those involving biphasic patterns of muscle activity. We propose that muscle synergies for locomotion are primarily organized within the spinal cord, although their full expression and proper timing requires inputs from supraspinal structures and/or limb afferents.

A Spinal Mechanism Related to Left-Right Symmetry Reduces Cutaneous Reflex Modulation Independently of Speed During Split-Belt Locomotion

Task- and phase-dependent reflex modulation during locomotion is well established, but we do not know the signals driving this modulation. To determine whether signals related to left-right symmetry of the locomotor pattern modulate cutaneous reflexes, we stimulated the superficial peroneal nerve in five intact female cats and in four spinal-transected cats (spinal cats, two males and two females) during split-belt locomotion at different left-right speeds. We compared cutaneous reflexes evoked in three ipsilateral and two contralateral hindlimb muscles during split-belt locomotion with those evoked during tied-belt (equal left-right speeds) locomotion at matched speeds of the slow and fast limbs. Our results showed similar phase-dependent modulation of cutaneous reflexes during tied-belt and split-belt locomotion in intact and spinal cats. During tied-belt locomotion in intact cats, an increase in speed significantly increased reflex modulation from minimum to maximum values, whereas in spinal cats, we observed a significant decrease. However, in all muscles of intact and spinal cats, split-belt locomotion significantly reduced reflex modulation compared with tied-belt locomotion independently of which limb was stepping on the slow or fast belt. Additionally, reflex modulation correlated more with spatial left-right symmetry, as opposed to a temporal one, in intact and spinal cats. Our results indicate that signals related to left-right symmetry reduce cutaneous reflex modulation independently of speed via a spinal mechanism. We propose that asymmetric sensory feedback from the left and right legs alters the state of the spinal network, thereby reducing cutaneous reflexes to prevent inputs from destabilizing a potentially unstable pattern.SIGNIFICANCE STATEMENT When we contact an obstacle during walking, receptors in the skin send signals to the CNS to alter the trajectory of the leg to maintain balance. This response, or reflex, is different when the leg is in the air and when it is contacting the ground. The reflex also differs when we walk at different speeds. Here, we investigated this reflex when the left and right legs were walking at different speeds on a split-belt treadmill in cats. We show that the reflex is smaller during split-belt locomotion compared with when both legs are walking at equal speeds. We propose that the spinal locomotor network controlling walking reduces the reflex response to optimize balance when gait is unstable.

Microstructural imaging in the spinal cord and validation strategies

In vivo histology using magnetic resonance imaging (MRI) is a newly emerging research field that aims to non-invasively characterize tissue microstructure. The implications of in vivo histology are many, from discovering novel biomarkers to studying human development, to providing tools for disease diagnosis and monitoring the effects of novel treatments on tissue. This review focuses on quantitative MRI (qMRI) techniques that are used to map spinal cord microstructure. Opening with a rationale for non-invasive imaging of the spinal cord, this article continues with a brief overview of the existing MRI techniques for axon and myelin imaging, followed by the specific challenges and potential solutions for acquiring and processing such data. The final part of this review focuses on histological validation, with suggested tissue preparation, acquisition and processing protocols for large-scale microscopy.

The Yoyo-Man

The paper reports on two results issued from a multidisciplinary research action exploring the motor synergies of anthropomorphic walking. By combining the biomechanical, neurophysiology, and robotics perspectives, it is intended to better understand human locomotion with the ambition to better design bipedal robot architectures. The motivation of the research starts from the simple observation that humans may stumble when following a simple reflex-based locomotion on uneven terrains. The rationale combines two well established results in robotics and neuroscience, respectively:

passive robot walkers, which are very efficient in terms of energy consumption, can be modeled by a simple rotating rimless wheel; humans and animals stabilize their head when moving.

The seminal hypothesis is then to consider a wheel equipped with a stabilized mass on top of it as a plausible model of bipedal walking. The two results presented in the paper support the hypothesis.

From a motion capture data basis of twelve human walkers, we show that the motions of the feet are organized around a geometric center, which is the center of mass, and is surprisingly not at the hip. After introducing a ground texture model that allows us to quantify the stability performance of walker control schemes, we show how compass-like passive walkers are better controlled when equipped with a stabilized 2-degree-of-freedom moving mass on top of them.

The center of mass and head then play complementary roles that define what we call the Yoyo-Man. Beyond the two results presented in the paper, the Yoyo-Man model opens new perspectives to explore the computational foundations of anthropomorphic walking.

Spike-timing-dependent plasticity in lower-limb motoneurons after human spinal cord injury

Recovery of lower-limb function after spinal cord injury (SCI) likely depends on transmission in the corticospinal pathway. Here, we examined whether paired corticospinal-motoneuronal stimulation (PCMS) changes transmission at spinal synapses of lower-limb motoneurons in humans with chronic incomplete SCI and aged-matched controls. We used 200 pairs of stimuli where corticospinal volleys evoked by transcranial magnetic stimulation (TMS) over the leg representation of the motor cortex were timed to arrive at corticospinal-motoneuronal synapses of the tibialis anterior (TA) muscle 2 ms before antidromic potentials evoked in motoneurons by electrical stimulation of the common peroneal nerve (PCMS+) or when antidromic potentials arrived 15 or 28 ms before corticospinal volleys (PCMS-) on separate days. Motor evoked potentials (MEPs) elicited by TMS and electrical stimulation were measured in the TA muscle before and after each stimulation protocol. After PCMS+, the size of MEPs elicited by TMS and electrical stimulation increased for up to 30 min in control and SCI participants. Notably, this was accompanied by increases in TA electromyographic activity and ankle dorsiflexion force in both groups, suggesting that this plasticity has functional implications. After PCMS-, MEPs elicited by TMS and electrical stimulation were suppressed if afferent input from the common peroneal nerve reduced TA MEP size during paired stimulation in both groups. In conclusion, PCMS elicits spike-timing-dependent changes at spinal synapses of lower-limb motoneurons in humans and has potential to improve lower-limb motor output following SCI.NEW & NOTEWORTHY Approaches that aim to enhance corticospinal transmission to lower-limb muscles following spinal cord injury (SCI) are needed. We demonstrate that paired corticomotoneuronal stimulation (PCMS) can enhance plasticity at spinal synapses of lower-limb motoneurons in humans with and without SCI. We propose that PCMS has potential for improving motor output in leg muscles in individuals with damage to the corticospinal tract.

The neural control of interlimb coordination during mammalian locomotion

Neuronal networks within the spinal cord directly control rhythmic movements of the arms/forelimbs and legs/hindlimbs during locomotion in mammals. For an effective locomotion, these networks must be flexibly coordinated to allow for various gait patterns and independent use of the arms/forelimbs. This coordination can be accomplished by mechanisms intrinsic to the spinal cord, somatosensory feedback from the limbs, and various supraspinal pathways. Incomplete spinal cord injury disrupts some of the pathways and structures involved in interlimb coordination, often leading to a disruption in the coordination between the arms/forelimbs and legs/hindlimbs in animal models and in humans. However, experimental spinal lesions in animal models to uncover the mechanisms coordinating the limbs have limitations due to compensatory mechanisms and strategies, redundant systems of control, and plasticity within remaining circuits. The purpose of this review is to provide a general overview and critical discussion of experimental studies that have investigated the neural mechanisms involved in coordinating the arms/forelimbs and legs/hindlimbs during mammalian locomotion.

Descending propriospinal neurons mediate restoration of locomotor function following spinal cord injury

Following spinal cord injury (SCI) in the lamprey, there is virtually complete recovery of locomotion within a few weeks, but interestingly, axonal regeneration of reticulospinal (RS) neurons is mostly limited to short distances caudal to the injury site. To explain this situation, we hypothesize that descending propriospinal (PS) neurons relay descending drive from RS neurons to indirectly activate spinal central pattern generators (CPGs). In the present study, the contributions of PS neurons to locomotor recovery were tested in the lamprey following SCI. First, long RS neuron projections were interrupted by staggered spinal hemitransections on the right side at 10% body length (BL; normalized from the tip of the oral hood) and on the left side at 30% BL. For acute recovery conditions (≤1 wk) and before axonal regeneration, swimming muscle burst activity was relatively normal, but with some deficits in coordination. Second, lampreys received two spaced complete spinal transections, one at 10% BL and one at 30% BL, to interrupt long-axon RS neuron projections. At short recovery times (3-5 wk), RS and PS neurons will have regenerated their axons for short distances and potentially established a polysynaptic descending command pathway. At these short recovery times, swimming muscle burst activity had only minor coordination deficits. A computer model that incorporated either of the two spinal lesions could mimic many aspects of the experimental data. In conclusion, descending PS neurons are a viable mechanism for indirect activation of spinal locomotor CPGs, although there can be coordination deficits of locomotor activity.

NEW & NOTEWORTHY

In the lamprey following spinal lesion-mediated interruption of long axonal projections of reticulospinal (RS) neurons, sensory stimulation still elicited relatively normal locomotor muscle burst activity, but with some coordination deficits. Computer models incorporating the spinal lesions could mimic many aspects of the experimental results. Thus, after disruption of long-axon projections from RS neurons in the lamprey, descending propriospinal (PS) neurons appear to be a viable compensatory mechanism for indirect activation of spinal locomotor networks.

Rehabilitation Strategies after Spinal Cord Injury: Inquiry into the Mechanisms of Success and Failure

Body-weight supported locomotor training (BWST) promotes recovery of load-bearing stepping in lower mammals, but its efficacy in individuals with a spinal cord injury (SCI) is limited and highly dependent on injury severity. While animal models with complete spinal transections recover stepping with step-training, motor complete SCI individuals do not, despite similarly intensive training. In this review, we examine the significant differences between humans and animal models that may explain this discrepancy in the results obtained with BWST. We also summarize the known effects of SCI and locomotor training on the muscular, motoneuronal, interneuronal, and supraspinal systems in human and non-human models of SCI and address the potential causes for failure to translate to the clinic. The evidence points to a deficiency in neuronal activation as the mechanism of failure, rather than muscular insufficiency. While motoneuronal and interneuronal systems cannot be directly probed in humans, the changes brought upon by step-training in SCI animal models suggest a beneficial re-organization of the systems' responsiveness to descending and afferent feedback that support locomotor recovery. The literature on partial lesions in humans and animal models clearly demonstrate a greater dependency on supraspinal input to the lumbar cord in humans than in non-human mammals for locomotion. Recent results with epidural stimulation that activates the lumbar interneuronal networks and/or increases the overall excitability of the locomotor centers suggest that these centers are much more dependent on the supraspinal tonic drive in humans. Sensory feedback shapes the locomotor output in animal models but does not appear to be sufficient to drive it in humans.

Interlimb Coordination during Tied-Belt and Transverse Split-Belt Locomotion before and after an Incomplete Spinal Cord Injury

Coordination between the arms/forelimbs and legs/hindlimbs is often impaired in humans and quadrupedal mammals after incomplete spinal cord injury. In quadrupeds, the forelimbs often take more steps than the hindlimbs, producing a two-to-one forelimb-hindlimb (2-1 FL-HL) coordination. In locomotor performance scales, this is generally considered a loss of FL-HL coordination. Here, FL-HL coordination was quantified before and 8 weeks after a lateral spinal hemisection at the sixth thoracic segment in six adult cats. Cats were tested during tied-belt locomotion (equal front and rear speeds) and transverse split-belt locomotion with the forelimbs or hindlimbs stepping faster. The results show that consistent phasing between forelimb and hindlimb movements was maintained after hemisection, even with the appearance of 2-1 FL-HL coordination, indicating that new stable forms of coordination emerge. Moreover, transverse split-belt locomotion potently modulated interlimb coordination and was capable of restoring a one-to-one FL-HL coordination with a faster treadmill speed for the hindlimbs. In conclusion, the results suggest that neural communication persists after an incomplete spinal cord injury, despite an unequal number of steps between the forelimbs and hindlimbs, and that interlimb coordination can be modulated by having the forelimbs or hindlimbs move at a faster frequency. We propose that locomotor recovery scales incorporate more sensitive methods to quantify FL-HL coordination, to better reflect residual functional capacity and possible cervicolumbar neural communication. Lastly, devising training protocols that make use of the bidirectional influences of the cervical and lumbar locomotor pattern generators could strengthen interlimb coordination and promote locomotor recovery.

Similar Motor Cortical Control Mechanisms for Precise Limb Control during Reaching and Locomotion

Throughout the course of evolution there has been a parallel development of the complexity and flexibility of the nervous system and the skeletomuscular system that it controls. This development is particularly evident for the cerebral cortical areas and the transformation of the use of the upper limbs from a purely locomotor function to one including, or restricted to, reaching and grasping. This study addresses the issue of whether the control of reaching has involved the development of new cortical circuits or whether the same neurons are used to control both locomotion and reaching. We recorded the activity of pyramidal tract neurons in the motor cortex of the cat both during voluntary gait modifications and during reaching. All cells showed generally similar patterns of activity in both tasks. More specifically, we showed that, in many cases, cells maintained a constant temporal relationship to the activity of synergistic muscle groups in each task. In addition, in some cells the relationship between the intensity of the cell discharge activity and the magnitude of the EMG activity was equally constant during gait modifications and reaching. As such, the results are compatible with the hypothesis that the corticospinal circuits used to control reaching evolved from those used to precisely modify gait.

Motor hypertonia and lack of locomotor coordination in mutant mice lacking DSCAM

Down syndrome cell adherence molecule (DSCAM) contributes to the normal establishment and maintenance of neural circuits. Whereas there is abundant literature regarding the role of DSCAM in the neural patterning of the mammalian retina, less is known about motor circuits. Recently, DSCAM mutation has been shown to impair bilateral motor coordination during respiration, thus causing death at birth. DSCAM mutants that survive through adulthood display a lack of locomotor endurance and coordination in the rotarod test, thus suggesting that the DSCAM mutation impairs motor control. We investigated the motor and locomotor functions of DSCAM(2J) mutant mice through a combination of anatomical, kinematic, force, and electromyographic recordings. With respect to wild-type mice, DSCAM(2J) mice displayed a longer swing phase with a limb hyperflexion at the expense of a shorter stance phase during locomotion. Furthermore, electromyographic activity in the flexor and extensor muscles was increased and coactivated over 20% of the step cycle over a wide range of walking speeds. In contrast to wild-type mice, which used lateral walk and trot at walking speed, DSCAM(2J) mice used preferentially less coordinated gaits, such as out-of-phase walk and pace. The neuromuscular junction and the contractile properties of muscles, as well as their muscle spindles, were normal, and no signs of motor rigidity or spasticity were observed during passive limb movements. Our study demonstrates that the DSCAM mutation induces dystonic hypertonia and a disruption of locomotor gaits.

[Central pattern generators in the spinal cord of the cat and their relevance in rehabilitation after spinal lesion]

The ability of the spinal cord to recover after partial or complete transection, and even reinitiate motor function, was investigated in several studies in cats. It has been shown that even after a complete spinalisation at the level of T12/T13, the possibility of restoration of hind-limb function is good. Central pattern generators (CPGs), located in the spinal cord, play an important role in this situation. Although CPGs alone are unable to restore function, the combination of CPGs with targeted and consistent mobility training and, in some cases, hind-limb sensory stimulation is essential to improve function. These result in a reorganisation of the CPGs and neuronal networks in the spinal cord. The age of the animal at the time of injury and the extent and localisation of lesions, play a crucial role in recovery. A new focus of research is the influence of neurotransmitters/neuromodulators on spinal-cord regeneration. How and to what extent these factors support locomotor training remains for further clinical investigation.

The "beneficial" effects of locomotor training after various types of spinal lesions in cats and rats

This chapter reviews a number of experiments on the recovery of locomotion after various types of spinal lesions and locomotor training mainly in cats. We first recall the major evidence on the recovery of hindlimb locomotion in completely spinalized cats at the T13 level and the role played by the spinal locomotor network, also known as the central pattern generator, as well as the beneficial effects of locomotor training on this recovery. Having established that hindlimb locomotion can recover, we raise the issue as to whether spinal plastic changes could also contribute to the recovery after partial spinal lesions such as unilateral hemisections. We found that after such hemisection at T10, cats could recover quadrupedal locomotion and that deficits could be improved by training. We further showed that, after a complete spinalization a few segments below the first hemisection (at T13, i.e., the level of previous studies on spinalization), cats could readily walk with the hindlimbs within hours of completely severing the remaining spinal tracts and not days as is usually the case with only a single complete spinalization. This suggests that neuroplastic changes occurred below the first hemisection so that the cat was already primed to walk after the spinalization subsequent to the hemispinalization 3 weeks before. Of interest is the fact that some characteristic kinematic features in trained or untrained hemispinalized cats could remain after complete spinalization, suggesting that spinal changes induced by training could also be durable. Other studies on reflexes and on the pattern of "fictive" locomotion recorded after curarization corroborate this view. More recent work deals with training cats in more demanding situations such as ladder treadmill (vs. flat treadmill) to evaluate how the locomotor training regimen can influence the spinal cord. Finally, we report our recent studies in rats using compressive lesions or surgical complete spinalization and find that some principles of locomotor recovery in cats also apply to rats when adequate locomotor training is provided.

Neurochemical excitation of thoracic propriospinal neurons improves hindlimb stepping in adult rats with spinal cord lesions

Using an in vitro neonatal rat brainstem-spinal cord preparation, we previously showed that cervicothoracic propriospinal neurons contribute to descending transmission of the bulbospinal locomotor command signal, and neurochemical excitation of these neurons facilitates signal propagation. The present study examined the relevance of these observations to adult rats in vivo. The first aim was to determine the extent to which rats are able to spontaneously recover hindlimb locomotor function in the presence of staggered contralateral hemisections (left T2-4 and right T9-11) designed to abolish all long direct bulbospinal projections. The second aim was to determine whether neurochemical excitation of thoracic propriospinal neurons in such animals facilitates hindlimb stepping. In the absence of intrathecal drug injection, all animals (n=24) displayed some degree of hindlimb recovery ranging from weak ankle movements to brief periods of unsupported hindlimb stepping on the treadmill. The effect of boluses of neurochemicals delivered via an intrathecal catheter (tip placed midway between the rostral and caudal thoracic hemisections) was examined at post-lesion weeks 3, 6 and 9. Quipazine was particularly effective facilitating hindlimb stepping. Subsequent complete transection above the rostral (n=3) or caudal (n=2) hemisections at week 9 had no consistent effect on drug-free locomotor performance, but the facilitatory effect of drug injection decreased in 4/5 animals. Two animals underwent complete transection at T3 as the first and only surgery and implantation of two intrathecal catheters targeted to the mid-thoracic and lumbar regions, respectively. A similar facilitatory effect on stepping was observed in response to drugs administered via either catheter. The results indicate that partial spontaneous recovery of stepping occurs in adult rats after abolishing all long direct bulbospinal connections, in contrast to previous studies suggesting that hindlimb stepping after dual hemisections either does not occur or is observed only if the second hemisection surgery is delayed relative to the first. The results support the hypothesis that artificial modulation of propriospinal neuron excitability may facilitate recovery of motor function after spinal cord injury. However, whether this facilitation is due to enhanced transmission of a descending locomotor signal or is the result of excitation of thoracolumbar circuits independent of supraspinal influence, requires further study.

Spatiotemporal control of interlimb coordination during transverse split-belt locomotion with 1:1 or 2:1 coupling patterns in intact adult cats

Interlimb coordination must be flexible to adjust to an ever-changing environment. Here adjustments in interlimb coordination were quantified during tied-belt (equal speed of the fore- and hindlimbs) and transverse split-belt (unequal speed of the fore- and hindlimbs) locomotion in five intact adult cats. Cats performed tied-belt locomotion at 0.4 m/s and 0.8 m/s. For transverse split-belt locomotion, the forelimbs stepped at 0.4 m/s and 0.8 m/s while the hindlimbs stepped at 0.8 m/s (4F8H condition) and 0.4 m/s (8F4H condition), respectively. In the 8F4H condition, the forelimbs could take two steps within one hindlimb cycle, or a 2:1 forelimb-hindlimb relationship. The sequence of limbs contacting the ground and the duration of support periods were differentially modified if the forelimbs stepped faster or slower than the hindlimbs. During transverse split-belt locomotion, the hindlimbs performed longer strides when the forelimbs took shorter strides. In the 8F4H condition with a 2:1 forelimb-hindlimb relationship, phase and gap intervals for the first and second steps were found around certain values and were not randomly distributed, indicating that a new coupling pattern was established. However, temporal and spatial coordination indexes revealed that bilateral coordination between hindlimbs was less accurate and more variable with a 2:1 coupling pattern. Importantly, the animals did not stumble, indicating that spatial and temporal adjustments in interlimb coordination allowed the animals to maintain dynamic stability. The results provide a better understanding of the spatiotemporal adjustments that take place among the four limbs during locomotion when interlimb coordination is challenged.

Treadmill training promotes spinal changes leading to locomotor recovery after partial spinal cord injury in cats

After a spinal hemisection at thoracic level in cats, the paretic hindlimb progressively recovers locomotion without treadmill training but asymmetries between hindlimbs persist for several weeks and can be seen even after a further complete spinal transection at T13. To promote optimal locomotor recovery after hemisection, such asymmetrical changes need to be corrected. In the present study we determined if the locomotor deficits induced by a spinal hemisection can be corrected by locomotor training and, if so, whether the spinal stepping after the complete spinal cord transection is also more symmetrical. This would indicate that locomotor training in the hemisected period induces efficient changes in the spinal cord itself. Sixteen adult cats were first submitted to a spinal hemisection at T10. One group received 3 wk of treadmill training, whereas the second group did not. Detailed kinematic and electromyographic analyses showed that a 3-wk period of locomotor training was sufficient to improve the quality and symmetry of walking of the hindlimbs. Moreover, after the complete spinal lesion was performed, all the trained cats reexpressed bilateral and symmetrical hindlimb locomotion within 24 h. By contrast, the locomotor pattern of the untrained cats remained asymmetrical, and the hindlimb on the side of the hemisection was still deficient. This study highlights the beneficial role of locomotor training in facilitating bilateral and symmetrical functional plastic changes within the spinal circuitry and in promoting locomotor recovery after an incomplete spinal cord injury.

A dual spinal cord lesion paradigm to study spinal locomotor plasticity in the cat

After a complete spinal cord injury (SCI) at the lowest thoracic level (T13), adult cats trained to walk on a treadmill can recover hindlimb locomotion within 2-3 weeks, resulting from the activity of a spinal circuitry termed the central pattern generator (CPG). The role of this spinal circuitry in the recovery of locomotion after partial SCIs, when part of descending pathways can still access the CPG, is not yet fully understood. Using a dual spinal lesion paradigm (first hemisection at T10 followed three weeks after by a complete spinalization at T13), we showed that major changes occurred in this locomotor spinal circuitry. These plastic changes at the spinal cord level could participate in the recovery of locomotion after partial SCI. This short review describes the main findings of this paradigm in adult cats.

Locomotor recovery after spinal cord hemisection/contusion injures in bonnet monkeys: footprint testing--a minireview

Spinal cord injuries usually produce loss or impairment of sensory, motor and reflex function below the level of damage. In the absence of functional regeneration or manipulations that promote regeneration, spontaneous improvements in motor functions occur due to the activation of multiple compensatory mechanisms in animals and humans following the partial spinal cord injury. Many studies were performed on quantitative evaluation of locomotor recovery after induced spinal cord injury in animals using behavioral tests and scoring techniques. Although few studies on rodents have led to clinical trials, it would appear imperative to use nonhuman primates such as macaque monkeys in order to relate the research outcomes to recovery of functions in humans. In this review, we will discuss some of our research evidences concerning the degree of spontaneous recovery in bipedal locomotor functions of bonnet monkeys that underwent spinal cord hemisection/contusion lesions. To our knowledge, this is the first report to discuss on the extent of spontaneous recovery in bipedal locomotion of macaque monkeys through the application of footprint analyzing technique. In addition, the results obtained were compared with the published data on recovery of quadrupedal locomotion of spinally injured rodents. We propose that the mechanisms underlying spontaneous recovery of functions in spinal cord lesioned monkeys may be correlated to the mature function of spinal pattern generator for locomotion under the impact of residual descending and afferent connections. Moreover, based on analysis of motor functions observed in locomotion in these subjected monkeys, we understand that spinal automatism and development of responses by afferent stimuli from outside the cord could possibly contribute to recovery of paralyzed hindlimbs. This report also emphasizes the functional contribution of progressive strengthening of undamaged nerve fibers through a collateral sprouts/synaptic plasticity formed in partially lesioned cord of monkeys.

Effect of locomotor training in completely spinalized cats previously submitted to a spinal hemisection

After a spinal hemisection in cats, locomotor plasticity occurring at the spinal level can be revealed by performing, several weeks later, a complete spinalization below the first hemisection. Using this paradigm, we recently demonstrated that the hemisection induces durable changes in the symmetry of locomotor kinematics that persist after spinalization. Can this asymmetry be changed again in the spinal state by interventions such as treadmill locomotor training started within a few days after the spinalization? We performed, in 9 adult cats, a spinal hemisection at thoracic level 10 and then a complete spinalization at T13, 3 weeks later. Cats were not treadmill trained during the hemispinal period. After spinalization, 5 of 9 cats were not trained and served as control while 4 of 9 cats were trained on the treadmill for 20 min, 5 d a week for 3 weeks. Using detailed kinematic analyses, we showed that, without training, the asymmetrical state of locomotion induced by the hemisection was retained durably after the subsequent spinalization. By contrast, training cats after spinalization induced a reversal of the left/right asymmetries, suggesting that new plastic changes occurred within the spinal cord through locomotor training. Moreover, training was shown to improve the kinematic parameters and the performance of the hindlimb on the previously hemisected side. These results indicate that spinal locomotor circuits, previously modified by past experience such as required for adaptation to the hemisection, can remarkably respond to subsequent locomotor training and improve bilateral locomotor kinematics, clearly showing the benefits of locomotor training in the spinal state.

Cells in the monkey ponto-medullary reticular formation modulate their activity with slow finger movements

Recent work has shown that the primate reticulospinal tract can influence spinal interneurons and motoneurons involved in control of the hand. However, demonstrating connectivity does not reveal whether reticular outputs are modulated during the control of different types of hand movement. Here, we investigated how single unit discharge in the pontomedullary reticular formation (PMRF) modulated during performance of a slow finger movement task in macaque monkeys. Two animals performed an index finger flexion–extension task to track a target presented on a computer screen; single units were recorded both from ipsilateral PMRF (115 cells) and contralateral primary motor cortex (M1, 210 cells). Cells in both areas modulated their activity with the task (M1: 87%, PMRF: 86%). Some cells (18/115 in PMRF; 96/210 in M1) received sensory input from the hand, showing a short-latency modulation in their discharge following a rapid passive extension movement of the index finger. Effects in ipsilateral electromyogram to trains of stimuli were recorded at 45 sites in the PMRF. These responses involved muscles controlling the digits in 13/45 sites (including intrinsic hand muscles, 5/45 sites). We conclude that PMRF may contribute to the control of fine finger movements, in addition to its established role in control of more proximal limb and trunk movements. This finding may be especially important in understanding functional recovery after brain lesions such as stroke.

Incomplete spinal cord injury promotes durable functional changes within the spinal locomotor circuitry

While walking in a straight path, changes in speed result mainly from adjustments in the duration of the stance phase while the swing phase remains relatively invariant, a basic feature of the spinal central pattern generator (CPG). To produce a broad range of locomotor behaviors, the CPG has to integrate modulatory inputs from the brain and the periphery and alter these swing/stance characteristics. In the present work we raise the issue as to whether the CPG can adapt or reorganize in response to a chronic change of supraspinal inputs, as is the case after spinal cord injury (SCI). Kinematic data obtained from six adult cats walking at different treadmill speeds were collected to calculate the cycle and subphase duration at different stages after a first spinal hemisection at T(10) and after a subsequent complete SCI at T(13) respectively aimed at disconnecting unilaterally and then totally the spinal cord from its supraspinal inputs. The results show, first, that the neural control of locomotion is flexible and responsive to a partial or total loss of supraspinal inputs. Second, we demonstrate that a hemisection induces durable plastic changes within the spinal locomotor circuitry below the lesion. In addition, this study gives new insights into the organization of the spinal CPG for locomotion such that phases of the step cycle (swing, stance) can be independently regulated for adapting to speed and also that the CPGs controlling the left and right hindlimbs can, up to a point, be regulated independently.

Recovery of hindlimb locomotion after incomplete spinal cord injury in the cat involves spontaneous compensatory changes within the spinal locomotor circuitry

After incomplete spinal cord injury (SCI), compensatory changes occur throughout the whole neuraxis, including the spinal cord below the lesion, as suggested by previous experiments using a dual SCI paradigm. Indeed, cats submitted to a lateral spinal hemisection at T10-T11 and trained on a treadmill for 3-14 wk re-expressed bilateral hindlimb locomotion as soon as 24 h after spinalization, a process that normally takes 2-3 wk when a complete spinalization is performed without a prior hemisection. In this study, we wanted to ascertain whether similar effects could occur spontaneously without training between the two SCIs and within a short period of 3 wk in 11 cats. One day after the complete spinalization, 9 of the 11 cats were able to re-express hindlimb locomotion either bilaterally (n = 6) or unilaterally on the side of the previous hemisection (n = 3). In these 9 cats, the hindlimb on the side of the previous hemisection (left hindlimb) performed better than the right side in contrast to that observed during the hemispinal period itself. Cats re-expressing the best bilateral hindlimb locomotion after spinalization had the largest initial hemilesion and the most prominent locomotor deficits after this first SCI. These results provide evidence that 1) marked reorganization of the spinal locomotor circuitry can occur without specific locomotor training and within a short period of 3 wk; 2) the spinal cord can reorganize in a more or less symmetrical way; and 3) the ability to walk after spinalization depends on the degree of deficits and adaptation observed in the hemispinal period.

Central Pattern Generators of the Mammalian Spinal Cord

Neuronal networks within the spinal cord of mammals are responsible for generating various rhythmic movements, such as walking, running, swimming, and scratching. The ability to generate multiple rhythmic movements highlights the complexity and flexibility of the mammalian spinal circuitry. The present review describes features of some rhythmic motor behaviors generated by the mammalian spinal cord and discusses how the spinal circuitry is able to produce different rhythmic movements with their own sets of goals and demands.

Changes in CNS structures after spinal cord lesions implications for BMI

It is well established that a spinal circuitry can generate locomotor movements of the hindlimbs in absence of descending supraspinal inputs. This is based, among others, on the observation that after a complete spinalization, cats can walk with the hindlimbs on a treadmill. Does this spinal pattern generator (CPG) also participate in the recovery of locomotion after a partial spinal cord lesion (SCI)? After such SCI, functional reorganization can occur spontaneously along the whole neuraxis, namely the spinal cord circuitry below the lesion (CPG) and in supraspinal structures still partially connected to the spinal cord. This review focuses mainly on the capacity of the spinal and supraspinal structures to reorganize spontaneously after incomplete SCI in animals (rats and cats). BMI approaches to foster recovery of functions after various types of SCI should take into account these changes at the various levels of the CNS.

Cortical control of adaptive locomotion in wild-type mice and mutant mice lacking the ephrin-Eph effector protein alpha2-chimaerin

In voluntary control, supraspinal motor systems select the appropriate response and plan movement mechanics to match task constraints. Spinal circuits translate supraspinal drive into action. We studied the interplay between motor cortex (M1) and spinal circuits during voluntary movements in wild-type (WT) mice and mice lacking the α2-chimaerin gene (Chn1(-/-)), necessary for ephrinB3-EphA4 signaling. Chn1(-/-) mice have aberrant bilateral corticospinal systems, aberrant bilateral-projecting spinal interneurons, and disordered voluntary control because they express a hopping gait, which may be akin to mirror movements. We addressed three issues. First, we determined the role of the corticospinal system in adaptive control. We trained mice to step over obstacles during treadmill locomotion. We compared performance before and after bilateral M1 ablation. WT mice adaptively modified their trajectory to step over obstacles, and M1 ablation increased substantially the incidence of errant steps over the obstacle. Chn1(-/-) mice randomly stepped or hopped during unobstructed locomotion but hopped over the obstacle. Bilateral M1 ablation eliminated this obstacle-dependent hop selection and increased forelimb obstacle contact errors. Second, we characterized the laterality of corticospinal action in Chn1(-/-) mice using pseudorabies virus retrograde transneuronal transport and intracortical microstimulation. We showed bilateral connections between M1 and forelimb muscles in Chn1(-/-) and unilateral connections in WT mice. Third, in Chn1(-/-) mice, we studied adaptive responses before and after unilateral M1 ablation. We identified a more important role for contralateral than ipsilateral M1 in hopping over the obstacle. Our findings suggest an important role for M1 in the mouse in moment-to-moment adaptive control, and further, using Chn1(-/-) mice, a role in mediating task-dependent selection of mirror-like hopping movements over the obstacle. Our findings also stress the importance of subcortical control during adaptive locomotion because key features of the trajectory remained largely intact after M1 ablation.

Dual spinal lesion paradigm in the cat: evolution of the kinematic locomotor pattern

The recovery of voluntary quadrupedal locomotion after an incomplete spinal cord injury can involve different levels of the CNS, including the spinal locomotor circuitry. The latter conclusion was reached using a dual spinal lesion paradigm in which a low thoracic partial spinal lesion is followed, several weeks later, by a complete spinal transection (i.e., spinalization). In this dual spinal lesion paradigm, cats can express hindlimb walking 1 day after spinalization, a process that normally takes several weeks, suggesting that the locomotor circuitry within the lumbosacral spinal cord had been modified after the partial lesion. Here we detail the evolution of the kinematic locomotor pattern throughout the dual spinal lesion paradigm in five cats to gain further insight into putative neurophysiological mechanisms involved in locomotor recovery after a partial spinal lesion. All cats recovered voluntary quadrupedal locomotion with treadmill training (3-5 days/wk) over several weeks. After the partial lesion, the locomotor pattern was characterized by several left/right asymmetries in various kinematic parameters, such as homolateral and homologous interlimb coupling, cycle duration, and swing/stance durations. When no further locomotor improvement was observed, cats were spinalized. After spinalization, the hindlimb locomotor pattern rapidly reappeared, but left/right asymmetries in swing/stance durations observed after the partial lesion could disappear or reverse. It is concluded that, after a partial spinal lesion, the hindlimb locomotor pattern was actively maintained by new dynamic interactions between spinal and supraspinal levels but also by intrinsic changes within the spinal cord.

Impaired transmission in the corticospinal tract and gait disability in spinal cord injured persons

Rehabilitation following spinal cord injury is likely to depend on recovery of corticospinal systems. Here we investigate whether transmission in the corticospinal tract may explain foot drop (inability to dorsiflex ankle) in persons with spinal cord lesion. The study was performed in 24 persons with incomplete spinal cord lesion (C1 to L1) and 15 healthy controls. Coherence in the 10- to 20-Hz frequency band between paired tibialis anterior muscle (TA) electromyographic recordings obtained in the swing phase of walking, which was taken as a measure of motor unit synchronization. It was significantly correlated with the degree of foot drop, as measured by toe elevation and ankle angle excursion in the first part of swing. Transcranial magnetic stimulation was used to elicit motor-evoked potentials (MEPs) in the TA. The amplitude of the MEPs at rest and their latency during contraction were correlated to the degree of foot drop. Spinal cord injured participants who exhibited a large foot drop had little or no MEP at rest in the TA muscle and had little or no coherence in the same muscle during walking. Gait speed was correlated to foot drop, and was the lowest in participants with no MEP at rest. The data confirm that transmission in the corticospinal tract is of importance for lifting the foot during the swing phase of human gait.

Afferent control of locomotor CPG: insights from a simple neuromechanical model

A simple neuromechanical model has been developed that describes a spinal central pattern generator (CPG) controlling the locomotor movement of a single-joint limb via activation of two antagonist (flexor and extensor) muscles. The limb performs rhythmic movements under control of the muscular, gravitational and ground reaction forces. Muscle afferents provide length-dependent (types Ia and II) and force-dependent (type Ib from the extensor) feedback to the CPG. We show that afferent feedback adjusts CPG operation to the kinematics and dynamics of the limb providing stable "locomotion." Increasing the supraspinal drive to the CPG increases locomotion speed by reducing the duration of stance phase. We show that such asymmetric, extensor-dominated control of locomotor speed (with relatively constant swing duration) is provided by afferent feedback independent of the asymmetric rhythmic pattern generated by the CPG alone (in "fictive locomotion" conditions). Finally, we demonstrate the possibility of reestablishing stable locomotion after removal of the supraspinal drive (associated with spinal cord injury) by increasing the weights of afferent inputs to the CPG, which is thought to occur following locomotor training.

Descending brain neurons in larval lamprey: spinal projection patterns and initiation of locomotion

In larval lamprey, partial lesions were made in the rostral spinal cord to determine which spinal tracts are important for descending activation of locomotion and to identify descending brain neurons that project in these tracts. In whole animals and in vitro brain/spinal cord preparations, brain-initiated spinal locomotor activity was present when the lateral or intermediate spinal tracts were spared but usually was abolished when the medial tracts were spared. We previously showed that descending brain neurons are located in eleven cell groups, including reticulospinal (RS) neurons in the mesenecephalic reticular nucleus (MRN) as well as the anterior (ARRN), middle (MRRN), and posterior (PRRN) rhombencephalic reticular nuclei. Other descending brain neurons are located in the diencephalic (Di) as well as the anterolateral (ALV), dorsolateral (DLV), and posterolateral (PLV) vagal groups. In the present study, the Mauthner and auxillary Mauthner cells, most neurons in the Di, ALV, DLV, and PLV cell groups, and some neurons in the ARRN and PRRN had crossed descending axons. The majority of neurons projecting in medial spinal tracts included large identified Müller cells and neurons in the Di, MRN, ALV, and DLV. Axons of individual descending brain neurons usually did not switch spinal tracts, have branches in multiple tracts, or cross the midline within the rostral cord. Most neurons that projected in the lateral/intermediate spinal tracts were in the ARRN, MRRN, and PRRN. Thus, output neurons of the locomotor command system are distributed in several reticular nuclei, whose neurons project in relatively wide areas of the cord.

Neurophysiologic and rehabilitation insights from the split-belt and other locomotor adaptation paradigms

Locomotion is incredibly flexible. Humans are able to stay upright and navigate long distances in the face of ever-changing environments and varied task demands, such as walking while carrying a heavy object or in thick mud. The focus of this review is a behavior that is critical for this flexibility: motor adaptation. Adaptation is defined here as the process of adjusting a movement to new demands through trial-and-error practice. A key feature of adaptation is that more practice without the new demand is required to return the movement to its original state. Thus, motor adaptation is a short-term motor learning process. Several studies have been undertaken to determine how humans adapt walking to novel circumstances. Many of these studies have examined locomotor adaptation using a split-belt treadmill. The results of these studies of people who were healthy and people with neurologic damage suggest that the cerebellum is required for normal adaptation of walking and that the role of cerebral structures may be less critical. They also suggest that intersegmental and interlimb coordination is critical but readily adaptable to accommodate changes in the environment. Locomotor adaptation also can be used to determine the walking potential of people with specific neurologic deficits. For instance, split-belt and limb-weighting locomotor adaptation studies show that adults with chronic stroke are capable of improving weight-bearing and spatiotemporal symmetry, at least temporarily. Our challenge as rehabilitation specialists is to intervene in ways that maximize this capacity.

Sensorimotor training promotes functional recovery and somatosensory cortical map reactivation following cervical spinal cord injury

Sensorimotor activity has been shown to play a key role in functional recovery after partial spinal cord injury (SCI). Most studies in rodents have focused on the rehabilitation of hindlimb locomotor functions after thoracic or lumbar SCI, whereas forelimb motor and somatosensory abilities after cervical SCI remain largely uninvestigated, despite the high incidence of such injuries in humans. Moreover, little is known about the neurophysiological substrates of training-induced recovery in supraspinal structures. This study was aimed at evaluating the effects of a training procedure combining both motor and sensory stimulation on behavioral performance and somatosensory cortical map remodeling after cervical (C4-C5) spinal hemisection in rats. This SCI severely impaired both sensory and motor capacities in the ipsilateral limbs. Without training, post-lesion motor capacities gradually improved, whereas forepaw tactile abilities remained impaired. Consistently, no stimulus-evoked responses were recorded within the forepaw representational zone in the primary somatosensory (S1) cortex at 2 months after the SCI. However, our data reveal that with training started from the 7th day post-lesion, a nearly complete recovery (characterized by an early and rapid improvement of motor functions) was associated with a gradual compensation of tactile deficits. Furthermore, the recovery of tactile abilities was correlated with the areal extent of reactivation of S1 cortex forepaw representations. Rehabilitative training promoted post-lesion adaptive plasticity, probably by enhancing endogenous activity within spared spinal and supraspinal circuits and pathways sustaining sensory and motor functions. This study highlights the beneficial effect of sensorimotor training in motor improvement and its critical influence on tactile recovery after SCI.

Reticulospinal neurons in the pontomedullary reticular formation of the monkey (Macaca fascicularis)

Recent neurophysiological studies indicate a role for reticulospinal neurons of the pontomedullary reticular formation (PMRF) in motor preparation and goal-directed reaching in the monkey. Although the macaque monkey is an important model for such investigations, little is known regarding the organization of the PMRF in the monkey. In the present study, we investigated the distribution of reticulospinal neurons in the macaque. Bilateral injections of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) were made into the cervical spinal cord. A wide band of retrogradely labeled cells was found in the gigantocellular reticular nucleus (Gi) and labeled cells continued rostrally into the caudal pontine reticular nucleus (PnC) and into the oral pontine reticular nucleus (PnO). Additional retrograde tracing studies following unilateral cervical spinal cord injections of cholera toxin subunit B revealed that there were more ipsilateral (60%) than contralateral (40%) projecting cells in Gi, while an approximately 50:50 ratio contralateral to ipsilateral split was found in PnC and more contralateral projections arose from PnO. Reticulospinal neurons in PMRF ranged widely in size from over 50 microm to under 25 microm across the major somatic axis. Labeled giant cells (soma diameters greater than 50 microm) comprised a small percentage of the neurons and were found in Gi, PnC and PnO. The present results define the origins of the reticulospinal system in the monkey and provide an important foundation for future investigations of the anatomy and physiology of this system in primates.

Re-expression of Locomotor Function After Partial Spinal Cord Injury

After a complete spinal section, quadruped mammals (cats, rats, and mice) can generally regain hindlimb locomotion on a treadmill because the spinal cord below the lesion can express locomotion through a neural circuitry termed the central pattern generator (CPG). In this review, we propose that the spinal CPG also plays a crucial role in the locomotor recovery after incomplete spinal cord injury.

Impairment of postural control in rabbits with extensive spinal lesions

Our previous studies on rabbits demonstrated that the ventral spinal pathways are of primary importance for postural control in the hindquarters. After ventral hemisection, postural control did not recover, whereas after dorsal or lateral hemisection it did. The aim of this study was to examine postural capacity of rabbits after more extensive lesion (3/4 section of the spinal cord at T(12) level), that is, with only one ventral quadrant spared (VQ animals). They were tested before (control) and after lesion on the platform periodically tilted in the frontal plane. In control animals, tilts of the platform regularly elicited coordinated electromyographic (EMG) responses in the hindlimbs, which resulted in generation of postural corrections and in maintenance of balance. In VQ rabbits, the EMG responses appeared only in a part of tilt cycles, and they could be either correctly or incorrectly phased in relation to tilts. Because of a reduced value and incorrect phasing of EMG responses on both sides, this muscle activity did not cause postural corrective movements in the majority of rabbits, and the body swayed together with the platform. In these rabbits, the ability to perform postural corrections did not recover during the whole period of observation (< or =30 days). Low probability of correct EMG responses to tilts in most rabbits as well as an appearance of incorrect responses to tilts suggest that the spinal reflex chains, necessary for postural control, have not been specifically selected by a reduced supraspinal drive transmitted via a single ventral quadrant.

Object avoidance during locomotion

Many animals rely on vision for navigating through complex environments and for avoiding specific obstacles during locomotion. Navigation and obstacle avoidance are tasks that depend on gathering information about the environment by vision and using this information at later times to guide limb and body movements. Here we review studies demonstrating the use of short-term visual memory during walking in humans and cats. Our own investigations have demonstrated that cats have the ability to retain a memory of an obstacle they have stepped over with the forelegs for many minutes and to use this memory to guide stepping of the hindlegs to avoid the remembered obstacle. A brain region that may be critically involved in the retention of memories of the location of obstacles is the posterior parietal cortex. Recordings from neurons in area 5 in the posterior parietal cortex in freely walking cats have revealed the existence of neurons whose activity is strongly correlated with the location of an obstacle relative to the body. How these neurons might be used to regulate motor commands remains to be established. We believe that studies on obstacle avoidance in walking cats have the potential to significantly advance our understanding of visuo-motor transformations. Current knowledge about the brain regions and pathways underlying visuo-motor transformations during walking are reviewed.

Effect of intrathecal administration of serotoninergic and noradrenergic drugs on postural performance in rabbits with spinal cord lesions

Our previous studies have shown that extensive spinal lesions at T12 in the rabbit [ventral hemisection (VHS) or 3/4-section that spares one ventral quadrant (VQ)] severely damaged the postural system. When tested on the platform periodically tilted in the frontal plane, VHS and VQ animals typically were not able to perform postural corrective movements by their hindlimbs, although EMG responses (correctly or incorrectly phased) could be observed. We attempted to restore postural control in VHS and VQ rabbits by applying serotoninergic and noradrenergic drugs to the spinal cord below the lesion through the intrathecal cannula. It was found that serotonin and quipazine (5-HT1,2,3 agonist) did not re-establish postural corrective movements. However, when applied during a 10-day period after lesion, these drugs produced a twofold increase of the proportion of correct EMG responses to tilts. It was also found that methoxamine (alpha1 noradrenergic agonist), as well as the mixture of methoxamine and quipazine, did not re-establish postural corrective movements and did not increase the proportion of correct EMG responses. Serotonin (at later stages) and methoxamine induced periodical bursting in EMGs, suggesting activation of spinal rhythm-generating networks. Appearance of bursting seems to perturb normal operation of postural mechanisms, as suggested by methoxamine-induced abolishment of postural effects of quipazine. When applied in an intact animal, none of the tested drugs affected the value of postural corrections or evoked periodical bursting. We conclude that activation of the serotoninergic system (but not the noradrenergic one) causes selective enhancement of spinal postural reflexes during the earlier postlesion period.

Prominent role of the spinal central pattern generator in the recovery of locomotion after partial spinal cord injuries

The re-expression of hindlimb locomotion after complete spinal cord injuries (SCIs) is caused by the presence of a spinal central pattern generator (CPG) for locomotion. After partial SCI, however, the role of this spinal CPG in the recovery of hindlimb locomotion in the cat remains mostly unknown. In the present work, we devised a dual-lesion paradigm to determine its possible contribution after partial SCI. After a partial section of the left thoracic segment T10 or T11, cats gradually recovered voluntary quadrupedal locomotion. Then, a complete transection was performed two to three segments more caudally (T13-L1) several weeks after the first partial lesion. Cats that received intensive treadmill training after the partial lesion expressed bilateral hindlimb locomotion within hours of the complete lesion. Untrained cats however showed asymmetrical hindlimb locomotion with the limb on the side of the partial lesion walking well before the other hindlimb. Thus, the complete spinalization revealed that the spinal CPG underwent plastic changes after the partial lesions, which were shaped by locomotor training. Over time, with further treadmill training, the asymmetry disappeared and a bilateral locomotion was reinstated. Therefore, although remnant intact descending pathways must contribute to voluntary goal-oriented locomotion after partial SCI, the recovery and re-expression of the hindlimb locomotor pattern mostly results from intrinsic changes below the lesion in the CPG and afferent inputs.