Functional organization within the medullary reticular formation of the intact unanesthetized cat. III. Microstimulation during locomotion

Fine-grained descending control of steering in walking Drosophila

Locomotion involves rhythmic limb movement patterns that originate in circuits outside the brain. Purposeful locomotion requires descending commands from the brain, but we do not understand how these commands are structured. Here we investigate this issue, focusing on the control of steering in walking Drosophila. First, we describe different limb "gestures" associated with different steering maneuvers. Next, we identify a set of descending neurons whose activity predicts steering. Focusing on two descending cell types downstream from distinct brain networks, we show that they evoke specific limb gestures: one lengthens strides on the outside of a turn, while the other attenuates strides on the inside of a turn. Notably, a single descending neuron can have opposite effects during different locomotor rhythm phases, and we identify networks positioned to implement this phase-specific gating. Together, our results show how purposeful locomotion emerges from brain cells that drive specific, coordinated modulations of low-level patterns.

A bi-cortical neuroprosthesis to modulate locomotion after incomplete spinal cord injury

Neuroprosthetic strategies seek to immediately alleviate deficits and reinstate voluntary control of movement. To facilitate recovery, it is crucial to gain a comprehensive understanding of the mechanisms involved in the return of intentional movement. Nevertheless, the precise relationship between the resurgence of cortical commands and the recovery of locomotion remains somewhat elusive. In the study conducted by Duguay, Bonizzato, Delivet-Mongrain, Fortier-Lebel and Martinez, we introduced a neuroprosthesis designed to deliver precise bi-cortical stimulation in a clinically relevant contusive spinal cord injury model. We conducted experiments in both healthy and spinal cord injured cats, where we fine-tuned the timing, duration, amplitude, and site of stimulation to modulate hindlimb locomotor output. In healthy cats, we observed a wide range of motor programs. However, after spinal cord injury, the induced hindlimb movements became highly stereotyped but were effective in modulating gait and reducing bilateral foot dragging. These results suggest that the neural basis for motor recovery traded off selectivity for effectiveness. Through a series of longitudinal assessments, we found that the restoration of locomotion following spinal cord injury was closely linked to the recovery of the descending neural drive. This underscores the importance of directing rehabilitation interventions toward the cortical target. The study results are discussed in terms of their impact and limitations.

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.

Recovery of walking in nonambulatory children with chronic spinal cord injuries: Case series

The immature central nervous system is recognized as having substantial neuroplastic capacity. In this study, we explored the hypothesis that rehabilitation can exploit that potential and elicit reciprocal walking in nonambulatory children with chronic, severe (i.e., lower extremity motor score < 10/50) spinal cord injuries (SCIs). Seven male subjects (3-12 years of age) who were at least 1-year post-SCI and incapable of discrete leg movements believed to be required for walking, enrolled in activity-based locomotor training (ABLT; clinicaltrials.gov NCT00488280). Six children completed the study. Following a minimum of 49 sessions of ABLT, three of the six children achieved walking with reverse rolling walkers. Stepping development, however, was not accompanied by improvement in discrete leg movements as underscored by the persistence of synergistic movements and little change in lower extremity motor scores. Interestingly, acoustic startle responses exhibited by the three responding children suggested preserved reticulospinal inputs to circuitry below the level of injury capable of mediating leg movements. On the other hand, no indication of corticospinal integrity was obtained with transcranial magnetic stimulation evoked responses in the same individuals. These findings suggest some children who are not predicted to improve motor and locomotor function may have a reserve of adaptive plasticity that can emerge in response to rehabilitative strategies such as ABLT. Further studies are warranted to determine whether a critical need exists to re-examine rehabilitation approaches for pediatric SCI with poor prognosis for any ambulatory recovery.

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.

The relationship between motor pathway damage and flexion-extension patterns of muscle co-excitation during walking

Background

Mass flexion-extension co-excitation patterns during walking are often seen as a consequence of stroke, but there is limited understanding of the specific contributions of different descending motor pathways toward their control. The corticospinal tract is a major descending motor pathway influencing the production of normal sequential muscle coactivation patterns for skilled movements. However, control of walking is also influenced by non-corticospinal pathways such as the corticoreticulospinal pathway that possibly contribute toward mass flexion-extension co-excitation patterns during walking. The current study sought to investigate the associations between damage to corticospinal (CST) and corticoreticular (CRP) motor pathways following stroke and the presence of mass flexion-extension patterns during walking as evaluated using module analysis.

Methods

Seventeen healthy controls and 44 stroke survivors were included in the study. We used non-negative matrix factorization for module analysis of paretic leg electromyographic activity. We typically have observed four modules during walking in healthy individuals. Stroke survivors often have less independently timed modules, for example two-modules presented as mass flexion-extension pattern. We used diffusion tensor imaging-based analysis where streamlines connecting regions of interest between the cortex and brainstem were computed to evaluate CST and CRP integrity. We also used a coarse classification tree analysis to evaluate the relative CST and CRP contribution toward module control.

Results

Interhemispheric CST asymmetry was associated with worse lower extremity Fugl-Meyer score (p = 0.023), propulsion symmetry (p = 0.016), and fewer modules (p = 0.028). Interhemispheric CRP asymmetry was associated with worse lower extremity Fugl-Meyer score (p = 0.009), Dynamic gait index (p = 0.035), Six-minute walk test (p = 0.020), Berg balance scale (p = 0.048), self-selected walking speed (p = 0.041), and propulsion symmetry (p = 0.001). The classification tree model reveled that substantial ipsilesional CRP or CST damage leads to a two-module pattern and poor walking ability with a trend toward increased compensatory contralesional CRP based control.

Conclusion

Both CST and CRP are involved with control of modules during walking and damage to both may lead to greater reliance on the contralesional CRP, which may contribute to a two-module pattern and be associated with worse walking performance.

Control of Mammalian Locomotion by Somatosensory Feedback

When animals walk overground, mechanical stimuli activate various receptors located in muscles, joints, and skin. Afferents from these mechanoreceptors project to neuronal networks controlling locomotion in the spinal cord and brain. The dynamic interactions between the control systems at different levels of the neuraxis ensure that locomotion adjusts to its environment and meets task demands. In this article, we describe and discuss the essential contribution of somatosensory feedback to locomotion. We start with a discussion of how biomechanical properties of the body affect somatosensory feedback. We follow with the different types of mechanoreceptors and somatosensory afferents and their activity during locomotion. We then describe central projections to locomotor networks and the modulation of somatosensory feedback during locomotion and its mechanisms. We then discuss experimental approaches and animal models used to investigate the control of locomotion by somatosensory feedback before providing an overview of the different functional roles of somatosensory feedback for locomotion. Lastly, we briefly describe the role of somatosensory feedback in the recovery of locomotion after neurological injury. We highlight the fact that somatosensory feedback is an essential component of a highly integrated system for locomotor control. © 2021 American Physiological Society. Compr Physiol 11:1-71, 2021.

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.

Common and distinct muscle synergies during level and slope locomotion in the cat

Although it is well established that the motor control system is modular, the organization of muscle synergies during locomotion and their change with ground slope are not completely understood. For example, typical reciprocal flexor-extensor muscle synergies of level walking in cats break down in downslope: one-joint hip extensors are silent throughout the stride cycle, whereas hindlimb flexors demonstrate an additional stance phase-related electromyogram (EMG) burst (Smith JL, Carlson-Kuhta P, Trank TV. J Neurophysiol 79: 1702-1716, 1998). Here, we investigated muscle synergies during level, upslope (27°), and downslope (-27°) walking in adult cats to examine common and distinct features of modular organization of locomotor EMG activity. Cluster analysis of EMG burst onset-offset times of 12 hindlimb muscles revealed five flexor and extensor burst groups that were generally shared across slopes. Stance-related bursts of flexor muscles in downslope were placed in a burst group from level and upslope walking formed by the rectus femoris. Walking upslope changed swing/stance phase durations of level walking but not the cycle duration. Five muscle synergies computed using non-negative matrix factorization accounted for at least 95% of variance in EMG patterns in each slope. Five synergies were shared between level and upslope walking, whereas only three of those were shared with downslope synergies; these synergies were active during the swing phase and phase transitions. Two stance-related synergies of downslope walking were distinct; they comprised a mixture of flexors and extensors. We suggest that the modular organization of muscle activity during level and slope walking results from interactions between motion-related sensory feedback, CPG, and supraspinal inputs.NEW & NOTEWORTHY We demonstrated that the atypical EMG activities during cat downslope walking, silent one-joint hip extensors and stance-related EMG bursts in flexors, have many features shared with activities of level and upslope walking. Majority of EMG burst groups and muscle synergies were shared among these slopes, and upslope modulated the swing/stance phase duration but not cycle duration. Thus, synergistic EMG activities in all slopes might result from a shared CPG receiving somatosensory and supraspinal inputs.

DSCAM Mutation Impairs Motor Cortex Network Dynamic and Voluntary Motor Functions

While it is well known that netrin-1 and its receptors UNC5 and UNC40 family members are involved in the normal establishment of the motor cortex and its corticospinal tract, less is known about its other receptor Down syndrome cell adherence molecule (DSCAM). DSCAM is expressed in the developing motor cortex, regulates axonal outgrowth of cortical neurons, and its mutation impairs the dendritic arborization of cortical neurons, thus suggesting that it might be involved in the normal development and functioning of the motor cortex. In comparison to WT littermates, DSCAM2J mutant mice slipped and misplaced their paw while walking on the rungs of a horizontal ladder, and exhibited more difficulties in stepping over an obstacle while walking at slow speed. Anterograde tracing showed a normal pyramidal decussation and corticospinal projection, but a more dorsal distribution of their axonal terminals in the spinal gray matter. Intracortical microstimulations showed a reduced corticospinal and intracortical efficacy, whereas stimulations of the pyramidal tract revealed a normal spinal efficacy and excitability of corticospinal tract axons, thus arguing for a dysfunctional cortical development. Our study reveals impairment of the network dynamics within the motor cortex, reducing corticospinal drive and impairing voluntary locomotor functions upon DSCAM2J mutation.

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.

Glutamatergic neurons of the gigantocellular reticular nucleus shape locomotor pattern and rhythm in the freely behaving mouse

Because of their intermediate position between supraspinal locomotor centers and spinal circuits, gigantocellular reticular nucleus (GRN) neurons play a key role in motor command. However, the functional contribution of glutamatergic GRN neurons in initiating, maintaining, and stopping locomotion is still unclear. Combining electromyographic recordings with optogenetic manipulations in freely behaving mice, we investigate the functional contribution of glutamatergic brainstem neurons of the GRN to motor and locomotor activity. Short-pulse photostimulation of one side of the glutamatergic GRN did not elicit locomotion but evoked distinct motor responses in flexor and extensor muscles at rest and during locomotion. Glutamatergic GRN outputs to the spinal cord appear to be gated according to the spinal locomotor network state. Increasing the duration of photostimulation increased motor and postural tone at rest and reset locomotor rhythm during ongoing locomotion. In contrast, photoinhibition impaired locomotor pattern and rhythm. We conclude that unilateral activation of glutamatergic GRN neurons triggered motor activity and modified ongoing locomotor pattern and rhythm.

Modularity speeds up motor learning by overcoming mechanical bias in musculoskeletal geometry

We can easily learn and perform a variety of movements that fundamentally require complex neuromuscular control. Many empirical findings have demonstrated that a wide range of complex muscle activation patterns could be well captured by the combination of a few functional modules, the so-called muscle synergies. Modularity represented by muscle synergies would simplify the control of a redundant neuromuscular system. However, how the reduction of neuromuscular redundancy through a modular controller contributes to sensorimotor learning remains unclear. To clarify such roles, we constructed a simple neural network model of the motor control system that included three intermediate layers representing neurons in the primary motor cortex, spinal interneurons organized into modules and motoneurons controlling upper-arm muscles. After a model learning period to generate the desired shoulder and/or elbow joint torques, we compared the adaptation to a novel rotational perturbation between modular and non-modular models. A series of simulations demonstrated that the modules reduced the effect of the bias in the distribution of muscle pulling directions, as well as in the distribution of torques associated with individual cortical neurons, which led to a more rapid adaptation to multi-directional force generation. These results suggest that modularity is crucial not only for reducing musculoskeletal redundancy but also for overcoming mechanical bias due to the musculoskeletal geometry allowing for faster adaptation to certain external environments.

Walking with perturbations: a guide for biped humans and robots

This paper provides an update on the neural control of bipedal walking in relation to bioinspired models and robots. It is argued that most current models or robots are based on the construct of a symmetrical central pattern generator (CPG). However, new evidence suggests that CPG functioning is basically asymmetrical with its flexor half linked more tightly to the rhythm generator. The stability of bipedal gait, which is an important problem for robots and biological systems, is also addressed. While it is not possible to determine how biological biped systems guarantee stability, robot solutions can be useful to propose new hypotheses for biology. In the second part of this review, the focus is on gait perturbations, which is an important topic in robotics in view of the frequent falls of robots when faced with perturbations. From the human physiology it is known that the initial reaction often consists of a brief interruption followed by an adequate response. For instance, the successful recovery from a trip is achieved using some basic reactions (termed elevating and lowering strategies), that depend on the phase of the step cycle of the trip occurrence. Reactions to stepping unexpectedly in a hole depend on comparing expected and real feedback. Implementation of these ideas in models and robotics starts to emerge, with the most advanced robots being able to learn how to fall safely and how to deal with complicated disturbances such as provided by walking on a split-belt.

Brainstem Steering of Locomotor Activity in the Newborn Rat

Control of locomotion relies on motor loops conveying modulatory signals between brainstem and spinal motor circuits. We investigated the steering control of the brainstem reticular formation over the spinal locomotor networks using isolated brainstem-spinal cord preparations of male and female neonatal rats. First, we performed patch-clamp recordings of identified reticulospinal cells during episodes of fictive locomotion. This revealed that a spinal ascending phasic modulation of reticulospinal cell activity is already present at birth. Half of the cells exhibited tonic firing during locomotion, while the other half emitted phasic discharges of action potentials phase locked to ongoing activity. We next showed that mimicking the phasic activity of reticulospinal neurons by applying patterned electrical stimulation bilaterally at the ventral caudal medulla level triggered fictive locomotion efficiently. Moreover, the brainstem stimuli-induced locomotor rhythm was entrained in a one-to-one coupling over a range of cycle periods (2-6 s). Additionally, we induced turning like motor outputs by either increasing or decreasing the relative duration of the stimulation trains on one side of the brainstem compared to the other. The ability of the patterned descending command to control the locomotor output depended on the functional integrity of ventral reticulospinal pathways and the involvement of local spinal central pattern generator circuitry. Altogether, this study provides a mechanism by which brainstem reticulospinal neurons relay steering and speed commands to the spinal locomotor networks.SIGNIFICANCE STATEMENT Locomotor function allows the survival of most animal species while sustaining the expression of fundamental behaviors. Locomotor activities adapt from moment to moment to behavioral and environmental changes. We show that the brainstem can control the spinal locomotor network outputs through phasic descending commands that alternate bilaterally. Manipulating the periodicity and/or the relative durations of the left and right descending commands at the brainstem level is efficient to set the locomotor speed and sustain directional changes.

Apraxia of gait- or apraxia of postural transitions?

"Apraxia of gait" is not a useful concept and freezing of gait should also not be considered an apraxia. The concept of apraxia may, however, be applied to distortions of postural transitions that can accompany fronto-parietal lesions.

Locomotor speed control circuits in the caudal brainstem

Locomotion is a universal behaviour that provides animals with the ability to move between places. Classical experiments have used electrical microstimulation to identify brain regions that promote locomotion, but the identity of neurons that act as key intermediaries between higher motor planning centres and executive circuits in the spinal cord has remained controversial. Here we show that the mouse caudal brainstem encompasses functionally heterogeneous neuronal subpopulations that have differential effects on locomotion. These subpopulations are distinguishable by location, neurotransmitter identity and connectivity. Notably, glutamatergic neurons within the lateral paragigantocellular nucleus (LPGi), a small subregion in the caudal brainstem, are essential to support high-speed locomotion, and can positively tune locomotor speed through inputs from glutamatergic neurons of the upstream midbrain locomotor region. By contrast, glycinergic inhibitory neurons can induce different forms of behavioural arrest mapping onto distinct caudal brainstem regions. Anatomically, descending pathways of glutamatergic and glycinergic LPGi subpopulations communicate with distinct effector circuits in the spinal cord. Our results reveal that behaviourally opposing locomotor functions in the caudal brainstem were historically masked by the unexposed diversity of intermingled neuronal subpopulations. We demonstrate how specific brainstem neuron populations represent essential substrates to implement key parameters in the execution of motor programs.

Rapid Alleviation of Parkinson's Disease Symptoms via Electrostimulation of Intrinsic Auricular Muscle Zones

Background: Deep brain stimulation of the subthalamic nucleus (STN-DBS) and the pedunculopontine nucleus (PPN) significantly improve cardinal motor symptoms and postural instability and gait difficulty, respectively, in Parkinson's disease (PD). Objective and Hypothesis: Intrinsic auricular muscle zones (IAMZs) allow the potential to simultaneously stimulate the C2 spinal nerve, the trigeminal nerve, the facial nerve, and sympathetic and parasympathetic nerves in addition to providing muscle feedback and control areas including the STN, the PPN and mesencephalic locomotor regions. Our aim was to observe the clinical responses to IAMZ stimulation in PD patients. Method: Unilateral stimulation of an IAMZ, which includes muscle fibers for proprioception, the facial nerve, and C2, trigeminal and autonomic nerve fibers, at 130 Hz was performed in a placebo- and sham-controlled, double-blinded, within design, two-armed study of 24 PD patients. Results: The results of the first arm (10 patients) of the present study demonstrated a substantial improvement in Unified Parkinson's Disease Ratings Scale (UPDRS) motor scores due to 10 min of IAMZ electrostimulation (p = 0.0003, power: 0.99) compared to the placebo control (p = 0.130). A moderate to large clinical difference in the improvement in UPDRS motor scores was observed in the IAMZ electrostimulation group. The results of the second arm (14 patients) demonstrated significant improvements with dry needling (p = 0.011) and electrostimulation of the IAMZ (p < 0.001) but not with sham electrostimulation (p = 0.748). In addition, there was a significantly greater improvement in UPDRS motor scores in the IAMZ electrostimulation group compared to the IAMZ dry needling group (p < 0.001) and the sham electrostimulation (p < 0.001) groups. The improvement in UPDRS motor scores of the IAMZ electrostimulation group (ΔUPDRS = 5.29) reached moderate to high clinical significance, which was not the case for the dry needling group (ΔUPDRS = 1.54). In addition, both arms of the study demonstrated bilateral improvements in motor symptoms in response to unilateral IAMZ electrostimulation. Conclusion: The present study is the first demonstration of a potential role of IAMZ electrical stimulation in improving the clinical motor symptoms of PD patients in the short term.

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.

Supraspinal Control Predicts Locomotor Function and Forecasts Responsiveness to Training after Spinal Cord Injury

Restoration of walking ability is an area of great interest in the rehabilitation of persons with spinal cord injury. Because many cortical, subcortical, and spinal neural centers contribute to locomotor function, it is important that intervention strategies be designed to target neural elements at all levels of the neuraxis that are important for walking ability. While to date most strategies have focused on activation of spinal circuits, more recent studies are investigating the value of engaging supraspinal circuits. Despite the apparent potential of pharmacological, biological, and genetic approaches, as yet none has proved more effective than physical therapeutic rehabilitation strategies. By making optimal use of the potential of the nervous system to respond to training, strategies can be developed that meet the unique needs of each person. To complement the development of optimal training interventions, it is valuable to have the ability to predict future walking function based on early clinical presentation, and to forecast responsiveness to training. A number of clinical prediction rules and association models based on common clinical measures have been developed with the intent, respectively, to predict future walking function based on early clinical presentation, and to delineate characteristics associated with responsiveness to training. Further, a number of variables that are correlated with walking function have been identified. Not surprisingly, most of these prediction rules, association models, and correlated variables incorporate measures of volitional lower extremity strength, illustrating the important influence of supraspinal centers in the production of walking behavior in humans.

What startles tell us about control of posture and gait

Recently, there has been an increase in studies evaluating startle reflexes and StartReact, many in tasks involving postural control and gait. These studies have provided important new insights. First, several experiments indicate a superimposition of startle reflex activity on the postural response during unexpected balance perturbations. Overlap in the expression of startle reflexes and postural responses emphasizes the possibility of, at least partly, a common substrate for these two types of behavior. Second, it is recognized that the range of behaviors, susceptible to StartReact, has expanded considerably. Originally this work was concentrated on simple voluntary ballistic movements, but gait initiation, online step adjustments and postural responses can be initiated earlier by a startling stimulus as well, indicating advanced motor preparation of posture and gait. Third, recent experiments on StartReact using TMS and patients with corticospinal lesions suggest that this motor preparation involves a close interaction between cortical and subcortical structures. In this review, we provide a comprehensive overview on startle reflexes, StartReact, and their interaction with posture and gait.

Differential modulation of descending signals from the reticulospinal system during reaching and locomotion

We tested the hypothesis that the same spinal interneuronal pathways are activated by the reticulospinal system during locomotion and reaching. If such were the case, we expected that microstimulation within the pontomedullary reticular formation (PMRF) would evoke qualitatively similar responses in muscles active during both behaviors. To test this, we stimulated in 47 sites within the PMRF during both tasks. Stimulation during locomotion always produced a strongly phase-dependent, bilateral pattern of activity in which activity in muscles was generally facilitated or suppressed during one phase of activity (swing or stance) and was unaffected in the other. During reaching, stimulation generally activated the same muscles as during locomotion, although the modulation of the magnitude of the evoked responses was less limb dependent than during locomotion. An exception was found for some forelimb flexor muscles that were strongly facilitated by stimulation during the swing phase of locomotion but were not influenced by stimulation during the transport phase of the reach. We suggest that during locomotion the activity in interneuronal pathways mediating signals from the reticulospinal system is subject to strong modulation by the central pattern generator for locomotion. During reach, we suggest that, for most muscles, the same spinal interneuronal pathways are used to modify muscle activity but are not as strongly gated according to limb use as during locomotion. Finally, we propose that the command for movement during discrete voluntary movements suppresses the influence of the reticulospinal system on selected forelimb flexor muscles, possibly to enhance fractionated control of movement.

Independent control of presynaptic inhibition by reticulospinal and sensory inputs at rest and during rhythmic activities in the cat

To be functionally relevant during movement, the transmission from primary afferents must be efficiently controlled by presynaptic inhibition. Sensory feedback, central pattern generators, and supraspinal structures can all evoke presynaptic inhibition, but we do not understand how these inputs interact during movement. Here, we investigated the convergence of inputs from the reticular formation and sensory afferents on presynaptic inhibitory pathways and their modulation at rest and during two fictive motor tasks (locomotion and scratch) in decerebrate cats. The amplitude of primary afferent depolarization (PAD), an estimate of presynaptic inhibition, was recorded in individual afferents with intra-axonal recordings and in a mix of afferents in lumbar dorsal rootlets (dorsal root potential [DRP]) with bipolar electrodes. There was no spatial facilitation between inputs from reticulospinal and sensory afferents with DRPs or PADs, indicating an absence of convergence. However, spatial facilitation could be observed by combining two sensory inputs, indicating that convergence was possible. Task-dependent changes in the amplitude of responses were similar for reticulospinal and sensory inputs, increasing during fictive locomotion and decreasing during fictive scratch. During fictive locomotion, DRP and PAD amplitudes evoked by reticulospinal inputs were increased during the flexion phase, whereas sensory-evoked DRPs and PADs showed maximal amplitude in either flexion or extension phases. During fictive scratch, the amplitudes of DRPs and PADs evoked by both sources were maximal in flexion. The absence of spatial facilitation and different phase-dependent modulation patterns during fictive locomotion are consistent with independent presynaptic inhibitory pathways for reticulospinal and sensory inputs.

The flexion synergy, mother of all synergies and father of new models of gait

Recently there has been a growing interest in the modular organization of leg movements, in particular those related to locomotion. One of the basic modules involves the flexion of the leg during swing and it was shown that this module is already present in neonates (Dominici et al., 2011). In this paper, we question how these finding build upon the original work by Sherrington, who proposed that the flexor reflex is the basic building block of flexion during swing phase. Similarly, the relation between the flexor reflex and the withdrawal reflex modules of Schouenborg and Weng (1994) will be discussed. It will be argued that there is large overlap between these notions on modules and the older concepts of reflexes. In addition, it will be shown that there is a great flexibility in the expression of some of these modules during gait, thereby allowing for a phase-dependent modulation of the appropriate responses. In particular, the end of the stance phase is a period when the flexor synergy is facilitated. It is proposed that this is linked to the activation of circuitry that is responsible for the generation of locomotor patterns (CPG, “central pattern generator”). More specifically, it is suggested that the responses in that period relate to the activation of a flexor burst generator. The latter structure forms the core of a new asymmetric model of the CPG. This activation is controlled by afferent input (facilitation by a broad range of afferents, suppression by load afferent input). Meanwhile, many of these physiologic features have found their way in the control of very flexible walking bipedal robots.

Motor deficits and recovery in rats with unilateral spinal cord hemisection mimic the Brown-Sequard syndrome

Cervical incomplete spinal cord injuries often lead to severe and persistent impairments of sensorimotor functions and are clinically the most frequent type of spinal cord injury. Understanding the motor impairments and the possible functional recovery of upper and lower extremities is of great importance. Animal models investigating motor dysfunction following cervical spinal cord injury are rare. We analysed the differential spontaneous recovery of fore- and hindlimb locomotion by detailed kinematic analysis in adult rats with unilateral C4/C5 hemisection, a lesion that leads to the Brown-Séquard syndrome in humans. The results showed disproportionately better performance of hindlimb compared with forelimb locomotion; hindlimb locomotion showed substantial recovery, whereas the ipsilesional forelimb remained in a very poor functional state. Such a differential motor recovery pattern is also known to occur in monkeys and in humans after similar spinal cord lesions. On the lesioned side, cortico-, rubro-, vestibulo- and reticulospinal tracts and the important modulatory serotonergic, dopaminergic and noradrenergic fibre systems were interrupted by the lesion. In an attempt to facilitate locomotion, different monoaminergic agonists were injected intrathecally. Injections of specific serotonergic and noradrenergic agonists in the chronic phase after the spinal cord lesion revealed remarkable, although mostly functionally negative, modulations of particular parameters of hindlimb locomotion. In contrast, forelimb locomotion was mostly unresponsive to these agonists. These results, therefore, show fundamental differences between fore- and hindlimb spinal motor circuitries and their functional dependence on remaining descending inputs and exogenous spinal excitation. Understanding these differences may help to develop future therapeutic strategies to improve upper and lower limb function in patients with incomplete cervical spinal cord injuries.

Involvement of the corticospinal tract in the control of human gait

Given the inherent mechanical complexity of human bipedal locomotion, and that complete spinal cord lesions in human leads to paralysis with no recovery of gait, it is often suggested that the corticospinal tract (CST) has a more predominant role in the control of walking in humans than in other animals. However, what do we actually know about the contribution of the CST to the control of gait? This chapter will provide an overview of this topic based on the premise that a better understanding of the role of the CST in gait will be essential for the design of evidence-based approaches to rehabilitation therapy, which will enhance gait ability and recovery in patients with lesions to the central nervous system (CNS). We review evidence for the involvement of the primary motor cortex and the CST during normal and perturbed walking and during gait adaptation. We will also discuss knowledge on the CST that has been gained from studies involving CNS lesions, with a particular focus on recent data acquired in people with spinal cord injury.

Measuring the motor output of the pontomedullary reticular formation in the monkey: do stimulus-triggered averaging and stimulus trains produce comparable results in the upper limbs?

The pontomedullary reticular formation (PMRF) of the monkey produces motor outputs to both upper limbs. EMG effects evoked from stimulus-triggered averaging (StimulusTA) were compared with effects from stimulus trains to determine whether both stimulation methods produced comparable results. Flexor and extensor muscles of scapulothoracic, shoulder, elbow, and wrist joints were studied bilaterally in two male M. fascicularis monkeys trained to perform a bilateral reaching task. The frequency of facilitation versus suppression responses evoked in the muscles was compared between methods. Stimulus trains were more efficient (94% of PMRF sites) in producing responses than StimulusTA (55%), and stimulus trains evoked responses from more muscles per site than from StimulusTA. Facilitation (72%) was more common from stimulus trains than StimulusTA (39%). In the overall results, a bilateral reciprocal activation pattern of ipsilateral flexor and contralateral extensor facilitation was evident for StimulusTA and stimulus trains. When the comparison was restricted to cases where both methods produced a response in a given muscle from the same site, agreement was very high, at 80%. For the remaining 20%, discrepancies were accounted for mainly by facilitation from stimulus trains when StimulusTA produced suppression, which was in agreement with the under-representation of suppression in the stimulus train data as a whole. To the extent that the stimulus train method may favor transmission through polysynaptic pathways, these results suggest that polysynaptic pathways from the PMRF more often produce facilitation in muscles that would typically demonstrate suppression with StimulusTA.

Speeding up gait initiation and gait-pattern with a startling stimulus

Human gait involves a repetitive leg motor pattern that emerges after gait initiation. While the automatic maintenance of the gait-pattern may be under the control of subcortical motor centres, gait initiation requires the voluntary launching of a different motor program. In this study, we sought to examine how the two motor programmes respond to an experimental manipulation of the timing of gait initiation. Subjects were instructed to start walking as soon as possible at the perception of an imperative signal (IS) that, in some interspersed trials was accompanied by a startling auditory stimulus (SAS). This method is known to shorten the latency for execution of the motor task under preparation. We reasoned that, if the two motor programmes were launched together, the gait-pattern sequence would respond to SAS in the same way as gait initiation. We recorded the gait phases and the electromyographic (EMG) activity of four muscles from the leg that initiates gait. In trials with SAS, latency of all gait initiation-related events showed a significant shortening and the bursts of EMG activity had higher amplitude and shorter duration than in trials without SAS. The events related to gait-pattern were also advanced but otherwise unchanged. The fact that all the effects of SAS were limited to gait initiation suggests that startle selectively can affect the neural structures involved in gait initiation. Additionally, the proportional advancement of the gait-pattern sequence to the end of gait initiation supports the view that gait initiation may actually trigger the inputs necessary for generating the gait-pattern sequence.

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.

The pontomedullary reticular formation contributes to the compensatory postural responses observed following removal of the support surface in the standing cat

This study was designed to determine the contribution of reticular neurons in the pontomedullary reticular formation (PMRF) to the postural responses produced to compensate for an unexpected perturbation. We recorded the activity of 48 neurons in the PMRF, including 41 reticulospinal neurons, to removal of the support surface under each of the four limbs in four cats. The perturbations produced robust postural responses that were divided into three periods: an initial postural response (P1) that displaced the center of vertical pressure over the two diagonal supporting limbs; a secondary response (P2) during which the cat restored a tripedal support pattern; and a prolonged tertiary response (P3) that maintained a stable posture over all three supporting limbs. Most (44/48) reticular neurons showed modified activity to perturbation of at least one limb and a majority (39/48) showed changes in activity to perturbations of more than one limb. A few (7/48) discharged to perturbations of all four limbs. Discharge frequency in neurons showing increased activity during P1 was relatively high (>100 Hz in 57% of the neurons responding to perturbations of either the left or right forelimbs, lFl and rFL) and of short latency (17 ms for the lFL and 14 ms for the rFL). Discharge activity in most neurons was sustained throughout P2 and P3 but at a reduced level. These data show that neurons in the PMRF discharge strongly in response to unexpected perturbations and in a manner consistent with a contribution to the compensatory responses that restore equilibrium.

Descending pathways in motor control

Each of the descending pathways involved in motor control has a number of anatomical, molecular, pharmacological, and neuroinformatic characteristics. They are differentially involved in motor control, a process that results from operations involving the entire motor network rather than from the brain commanding the spinal cord. A given pathway can have many functional roles. This review explores to what extent descending pathways are highly conserved across species and concludes that there are actually rather widespread species differences, for example, in the transmission of information from the corticospinal tract to upper limb motoneurons. The significance of direct, cortico-motoneuronal (CM) connections, which were discovered a little more than 50 years ago, is reassessed. I conclude that although these connections operate in parallel with other less direct linkages to motoneurons, CM influence is significant and may subserve some special functions including adaptive motor behaviors involving the distal extremities.

Neurons in the pontomedullary reticular formation signal posture and movement both as an integrated behavior and independently

We have previously suggested that the discharge characteristics of some neurons in the pontomedullary reticular formation (PMRF) are contingent on the simultaneous requirement for activity in both ipsilateral flexor muscles and contralateral extensors. To test this hypothesis we trained cats to stand on four force platforms and to perform a task in which they were required to reach forward with one forelimb or the other and depress a lever. As such the task required the cat to make a flexion movement followed by an extension in the reaching limb while maintaining postural support by increasing extensor muscle tonus in the supporting limbs. We recorded the activity of 131 neurons from the PMRF of three cats during left, ipsilateral reach. Of these, 86/131 (66%) showed a change in discharge frequency prior to the onset of activity in one of the prime flexor muscles and 43/86 (50%) showed a bimodal pattern of discharge in which activity decreased during the lever press. Among the remaining cells, 28/86 (33%) showed maintained activity throughout the reach and the lever press. Most cells showed a broadly similar pattern of discharge during reaches with the right, contralateral limb. We suggest these results support the view that a population of neurons within the PMRF contributes to the control of movement in one forelimb and the control of posture in the other forelimb as a coordinated unit. Another population of neurons contributes to the control of postural support independently of the nature of the activity in the reaching limb.

Uncrossed actions of feline corticospinal tract neurones on lumbar interneurones evoked via ipsilaterally descending pathways

Effects of stimulation of ipsilateral pyramidal tract (PT) fibres were analysed in interneurones in midlumbar segments of the cat spinal cord in search of interneurones mediating disynaptic actions of uncrossed PT fibres on hindlimb motoneurones. The sample included 44 intermediate zone and ventral horn interneurones, most with monosynaptic input from group I and/or group II muscle afferents and likely to be premotor interneurones. Monosynaptic EPSPs evoked by stimulation of the ipsilateral PT were found in 12 of the 44 (27%) interneurones, while disynaptic or trisynaptic EPSPs were evoked in more than 75%. Both appeared at latencies that were either longer or within the same range as those of disynaptic EPSPs and IPSPs evoked by PT stimuli in motoneurones, making it unlikely that premotor interneurones in pathways from group I and/or II afferents relay the earliest actions of uncrossed PT fibres on motoneurones. These interneurones might nevertheless contribute to PT actions at longer latencies. Uncrossed PT actions on interneurones were to a great extent relayed via reticulospinal neurones with axons in the ipsilateral medial longitudinal fascicle (MLF), as indicated by occlusion and mutual facilitation of actions evoked by PT and MLF stimulation. However, PT actions were also relayed by other supraspinal or spinal neurones, as some remained after MLF lesions. Mutual facilitation and occlusion of actions evoked from the ipsilateral and contralateral PTs lead to the conclusion that the same midlumbar interneurones in pathways from group I or II muscle afferents may relay uncrossed and crossed PT actions.

Plasticity of connections underlying locomotor recovery after central and/or peripheral lesions in the adult mammals

This review discusses some aspects of plasticity of connections after spinal injury in adult animal models as a basis for functional recovery of locomotion. After reviewing some pitfalls that must be avoided when claiming functional recovery and the importance of a conceptual framework for the control of locomotion, locomotor recovery after spinal lesions, mainly in cats, is summarized. It is concluded that recovery is partly due to plastic changes within the existing spinal locomotor networks. Locomotor training appears to change the excitability of simple reflex pathways as well as more complex circuitry. The spinal cord possesses an intrinsic capacity to adapt to lesions of central tracts or peripheral nerves but, as a rule, adaptation to lesions entails changes at both spinal and supraspinal levels. A brief summary of the spinal capacity of the rat, mouse and human to express spinal locomotor patterns is given, indicating that the concepts derived mainly from work in the cat extend to other adult mammals. It is hoped that some of the issues presented will help to evaluate how plasticity of existing connections may combine with and potentiate treatments designed to promote regeneration to optimize remaining motor functions.

Descending signals from the pontomedullary reticular formation are bilateral, asymmetric, and gated during reaching movements in the cat

We examined the contribution of neurons within the pontomedullary reticular formation (PMRF) to the control of reaching movements in the cat. We recorded the activity of 127 reticular neurons, including 56 reticulospinal neurons, during movements of each forelimb; 67/127 of these neurons discharged prior to the onset of activity in the prime flexor muscles during the reach of the ipsilateral limb and form the focus of this report. Most neurons (63/67) showed similar patterns and levels of discharge activity during reaches of either limb, although activity was slightly greater during reach of the ipsilateral limb. In 26/67 cells, the initial change in discharge activity was time-locked to the go signal during reaches of either limb; we have argued that this early discharge contributes to the anticipatory postural adjustments that precede movement. In 11/26 cells, the initial change in activity was reciprocal for reaches with the left and right limbs, although activity during the movement was nonreciprocal. Spike-triggered averaging produced postspike facilitation or depression (PSD) in 12/50 cells during reaches of the limb ipsilateral to the recording site and in 17/49 cells during reach of the contralateral limb. Some cells produced PSD in ipsilateral extensor muscles before the start of the reach and during reaches made with the contralateral, but not the ipsilateral limb; this suggests the signal must be differentially gated. Overall, the results suggest a strong bilateral, albeit asymmetric, contribution from the PMRF to the control of posture and movement during voluntary movement.

How can corticospinal tract neurons contribute to ipsilateral movements? A question with implications for recovery of motor functions

In this review, the authors discuss some recent findings that bear on the issue of recovery of function after corticospinal tract lesions. Conventionally the corticospinal tract is considered to be a crossed pathway, in keeping with the clinical findings that damage to one hemisphere, for example, in stroke, leads to a contralateral paresis and, if the lesion is large, a paralysis. However, there has been great interest in the possibility of compensatory recovery of function using the undamaged hemisphere. There are several substrates for this including ipsilaterally descending corticospinal fibers and bilaterally operating neuronal networks. Recent studies provide important evidence bearing on both of these issues. In particular, they reveal networks of neurons interconnecting two sides of the gray matter at both brainstem and spinal levels, as well as intrahemispheric transcallosal connections. These may form "detour circuits" for recovery of function, and here the authors will consider some possibilities for exploiting these networks for motor control, even though their analysis is still at an early stage.

Bilateral actions of the reticulospinal tract on arm and shoulder muscles in the monkey: stimulus triggered averaging

The motor output of the pontomedullary reticular formation (PMRF) was investigated to determine the reticulospinal system's capacity for bilateral control of the upper limbs. Stimulus triggered electromyographic averages (StimulusTA) were constructed from muscles of both upper limbs while two awake monkeys (Macaca fascicularis) performed a reaching task using either arm. Extensor and flexor muscles were studied at the wrist, elbow, and shoulder; muscles acting on the scapula were also studied. Post-stimulus effects (PStEs) resulted from 435 (81%) of 535 sites tested. Of 1611 PStEs analyzed, 58% were post-stimulus suppression (PStS), and 42% were post-stimulus facilitation (PStF). Onset latency was earlier for PStF than PStS, duration was longer for PStS, and amplitude was larger for PStF. Ipsilateral and contralateral PStEs were equally prevalent; bilateral responses were typical. In the ipsilateral forelimb and shoulder, the prevalent pattern was flexor PStF and extensor PStS; the opposite pattern was prevalent contralaterally. Sites producing strong ipsilateral upper trapezius PStF were concentrated in a region caudal and ventral to abducens. The majority of muscles studied had no clear somatotopic organization. Overall, the results indicate the monkey PMRF has the capacity to support bilateral coordination of limb movements using reciprocal actions within a limb and between sides.

Differential responsiveness of c-Fos expression in the rat medulla oblongata to different treadmill running speeds

Expression of the inducible transcription factor c-Fos was mapped in the rat medulla oblongata to identify the brain areas respond to different running speeds. Rats were subjected to 30 min of running, either at high speed, low speed or just sitting on a treadmill (control). Blood lactate levels were measured to confirm the physiological impact of different exercise intensities. The number of c-Fos-ir cells was counted and their spatial distributions were mapped through the rostral to the caudal level in the medulla. A statistically significant exercise intensity-dependent induction of c-Fos was observed in the nucleus of the solitary tract (NTS) and caudal ventrolateral medulla (CVL) in the medulla. Further, c-Fos induction was more predominant in the caudal part of each nucleus. The present data clearly show that different running speeds cause differential activation of each nucleus in the medulla, and in particular, the caudal parts of the NTS and the CVL are the most responsive to speed changes. The present study identifies brain areas newly found to be responsive to changes in running speed. These findings are likely to be particularly helpful in studies of specific neural circuits and their functions in response to different running speeds.

Startle responses in Parkinson patients during human gait

Falls frequently occur in patients with Parkinson's disease (Bloem et al. 2001). One potential source for such falls during walking might be caused by the reaction to loud noises. In normal subjects startle reactions are well integrated in the locomotor activity (Nieuwenhuijzen et al. 2000), but whether this is also achieved in Parkinson patients is unknown. Therefore, in the present study, the startle response during walking was studied in eight patients with Parkinson's disease and in eight healthy subjects. To examine how startle reactions are incorporated in an ongoing gait pattern of these patients, unexpected auditory stimuli were presented in six phases of the step cycle during walking on a treadmill. For both legs electromyographic activity was recorded from biceps femoris and tibialis anterior. In addition, we measured the stance and swing phases of both legs, along with the knee angles of both legs and the left ankle angle. In all subjects and all muscles, responses were detected. The pattern of the responses, latency, duration, and phase-dependent modulation was similar in both groups. However, the mean response amplitude was larger in patients due to a smaller habituation rate. No correlation was found between the degree of habituation and disease severity. Moreover, a decreased habituation was already observed in mildly affected patients, indicating that habituation of the startle response is a sensitive measure of Parkinson's disease. The results complement the earlier findings of reduced habituation of blink responses in Parkinson's disease. With respect to behavioral changes in healthy subjects we observed that startle stimuli induced a shortening of the step cycle and a decrease in range of motion. In the patient group, less shortening of the subsequent step cycle and no decrease in range of motion of the knee and ankle was seen. It is argued that the observed changes might contribute to the high incidence of falls in patients with Parkinson's disease.

Contribution of the motor cortex to the structure and the timing of hindlimb locomotion in the cat: a microstimulation study

We used microstimulation to examine the contribution of the motor cortex to the structure and timing of the hindlimb step cycle during locomotion in the intact cat. Stimulation was applied to the hindlimb representation of the motor cortex in 34 sites in three cats using either standard glass-insulated microelectrodes (16 sites in 1 cat) or chronically implanted microwire electrodes (18 sites in 2 cats). Stimulation at just suprathreshold intensities with the cat at rest produced multi-joint movements at a majority of sites (21/34, 62%) but evoked responses restricted to a single joint, normally the ankle, at the other 13/34 (38%) sites. Stimulation during locomotion generally evoked larger responses than the same stimulation at rest and frequently activated additional muscles. Stimulation at all 34 sites evoked phase-dependent responses in which stimulation in swing produced transient increases in activity in flexor muscles while stimulation during stance produced transient decreases in activity in extensors. Stimulation with long (200 ms) trains of stimuli in swing produced an increased level of activity and duration of flexor muscles without producing changes in cycle duration. In contrast, stimulation during stance decreased the duration of the extensor muscle activity and initiated a new and premature period of swing, resetting the step cycle. Stimulation of the pyramidal tract in two of these three cats as well as in two additional ones produced similar effects. The results show that the motor cortex is capable of influencing hindlimb activity during locomotion in a similar manner to that seen for the forelimb.

Adaptive changes of locomotion after central and peripheral lesions

This paper reviews findings on the adaptive changes of locomotion in cats after spinal cord or peripheral nerve lesions. From the results obtained after lesions of the ventral/ventrolateral pathways or the dorsal/dorsolateral pathways, we conclude that with extensive but partial spinal lesions, cats can regain voluntary quadrupedal locomotion on a treadmill. Although tract-specific deficits remain after such lesions, intact descending tracts can compensate for the lesioned tracts and access the spinal network to generate voluntary locomotion. Such neuroplasticity of locomotor control mechanisms is also demonstrated after peripheral nerve lesions in cats with intact or lesioned spinal cords. Some models have shown that recovery from such peripheral nerve lesions probably involves changes at the supra spinal and spinal levels. In the case of somesthesic denervation of the hindpaws, we demonstrated that cats with a complete spinal section need some cutaneous inputs to walk with a plantigrade locomotion, and that even in this spinal state, cats can adapt their locomotion to partial cutaneous denervation. Altogether, these results suggest that there is significant plasticity in spinal and supraspinal locomotor controls to justify the beneficial effects of early proactive and sustained locomotor training after central (Rossignol and Barbeau 1995; Barbeau et al. 1998) or peripheral lesions.Key words: spinal lesions, nerve lesions, locomotion, neuroplisticity, locomotor training.

Movement-related and preparatory activity in the reticulospinal system of the monkey

Three monkeys ( M. fascicularis) performed a center-out, two-dimensional reaching task that included an instructed delay interval based on a color-coded visuospatial cue. Neural activity in the medial pontomedullary reticular formation (mPMRF) was recorded along with hand movement. Of 176 neurons with movement-related activity, 109 (62%) had movement-related but not preparatory activity (M cells), and 67 (38%) had both movement-related and preparatory activity (MP cells). EOG analyses indicated that the preparatory activity was not consistent with control of eye movements. There were slight changes in electromyograms (EMG) late in the instructed delay period before the Go cue, but these were small compared with the movement-related EMG activity. Preparatory activity, like the EMG activity, was also confined to the end of the instructed delay period for 14 MP cells, but the remaining 53 MP cells (30%) had preparatory activity that was not reflected in the EMG. Peri-movement neural activity varied with movement direction for 70% of the cells, but this variation rarely fit circular statistics commonly used for studies of directional tuning; directional tuning was even less common in the preparatory activity. These data show that neurons in the mPMRF are strongly modulated during small reaching movements, but this modulation was rarely correlated with the trajectory of the hand. In accord with findings in the literature from other regions of the CNS, evidence of activity related to motor preparation in these cells indicates that this function is distributed in the nervous system and is not a feature limited to the cerebral cortex.

Independent and convergent signals from the pontomedullary reticular formation contribute to the control of posture and movement during reaching in the cat

We have addressed the nature of the postural control signals contained within the discharge activity of neurons in the pontomedullary reticular formation, including reticulospinal neurons, during a reaching task in the cat. We recorded the activity of 142 neurons during ipsilateral reaching movements that required anticipatory postural adjustments (APAs) in the supporting limbs to maintain equilibrium. Discharge activity in 82/142 (58%) neurons was significantly increased before the onset of the reach. Most of these neurons discharged either in a phasic (22/82), tonic (10/82), or phasic/tonic (41/82) pattern. In each of these 3 groups, the onset of the discharge activity in some neurons was temporally related either to the go signal or to the onset of the movement. In many neurons, one component of the discharge sequence was better related to the go signal and another to the onset of the movement. Based on our previous behavioral study during the same task, we suggest that reticular neurons in which the discharge activity is better related to the go signal contribute to the initiation of the APAs that precede the movement. Neurons in which the discharge activity is better related to the movement signal might contribute to the initiation of the movement and to the production of the postural responses that accompany that movement. Together our results suggest the existence of neurons that signal posture and movement independently and others that encode a convergent signal that contributes to the control of both posture and movement.

Motor outputs from the primate reticular formation to shoulder muscles as revealed by stimulus-triggered averaging

The motor output of the medial pontomedullary reticular formation (mPMRF) was investigated using stimulus-triggered averaging (StimulusTA) of EMG responses from proximal arm and shoulder muscles in awake, behaving monkeys (M. fascicularis). Muscles studied on the side ipsilateral (i) to stimulation were biceps (iBic), triceps (iTri), anterior deltoid (iADlt), posterior deltoid (iPDlt), and latissimus dorsi (iLat). The upper and middle trapezius were studied on the ipsilateral and contralateral (c) side (iUTr, cUTr, iMTr, cMTr). Of 133 sites tested, 97 (73%) produced a poststimulus effect (PStE) in one or more muscles; on average, 38% of the sampled muscles responded per effective site. For responses that were observed in the arm and shoulder, poststimulus facilitation (PStF) was prevalent for the flexors, iBic (8 responses, 100% PStF) and iADlt (13 responses, 77% PStF), and poststimulus suppression (PStS) was prevalent for the extensors, iTri (22 responses, 96% PStS) and iLat (16 responses, 81% PStS). For trapezius muscles, PStS of upper trapezius (iUTr, 49 responses, 73% PStS) and PStF of middle trapezius (iMTr, 22 responses, 64% PStF) were prevalent ipsilaterally, and PStS of middle trapezius (cMTr, 6 responses, 67% PStS) and PStF of upper trapezius (cUTr, 46 responses, 83% PStS) were prevalent contralaterally. Onset latencies were significantly earlier for PStF (7.0 +/- 2.2 ms) than for PStS (8.6 +/- 2.0 ms). At several sites, extremely strong PStF was evoked in iUTr, even though PStS was most common for this muscle. The anatomical antagonists iBic/iTri were affected reciprocally when both responded. The bilateral muscle pair iUTr/cUTr demonstrated various combinations of effects, but cUTr PStF with iUTr PStS was prevalent. Overall, the results are consistent with data from the cat and show that outputs from the mPMRF can facilitate or suppress activity in muscles involved in reaching; responses that would contribute to flexion of the ipsilateral arm were prevalent.

Cortical and brainstem control of locomotion

While a basic locomotor rhythm is centrally generated by spinal circuits, descending pathways are critical for ensuring appropriate anticipatory modifications of gait to accommodate uneven terrain. Neurons in the motor cortex command the changes in muscle activity required to modify limb trajectory when stepping over obstacles. Simultaneously, neurons in the brainstem reticular formation ensure that these modifications are superimposed on an appropriate base of postural support. Recent experiments suggest that the same neurons in the same structures also provide similar information during reaching movements. It is suggested that, during both locomotion and reaching movements, the final expression of descending signals is influenced by the state and excitability of the spinal circuits upon which they impinge.