Neural bases of postural control

Generalizability of foot placement control strategies during unperturbed and perturbed gait

Control of foot placement is an essential strategy for maintaining balance during walking. During unperturbed, steady-state walking, foot placement can be accurately described as a linear function of the body's centre of mass (CoM) state at midstance. However, it is uncertain if this mapping from CoM state to foot placement generalizes to larger perturbations that could potentially cause falls. Recovery from these perturbations may require reactive control strategies not observed during unperturbed walking. Here, we used unpredictable changes in treadmill belt speed to assess the generalizability of foot placement mappings identified during unperturbed walking. We found that foot placement mappings generalized poorly from unperturbed to perturbed walking and differed for forward perturbation versus backward perturbation. We also used the singular value decomposition of the mapping matrix to reveal that people were more sensitive to backward versus forward perturbations. Together, these results indicate that a single linear mapping cannot describe the foot placement control during both forward and backward losses of balance induced by treadmill belt speed perturbations. Better characterization of human balance control strategies could improve our understanding of why different neuromotor disorders result in heightened fall risk and inform the design of controllers for balance-assisting devices.

Standing Reactive Postural Responses of Lower Limbs With and Without Self-Balance Assistance in Individuals With Spinal Cord Injury Receiving Epidural Stimulation

Spinal cord epidural stimulation can promote the recovery of motor function in individuals with severe spinal cord injury (SCI) by enabling the spinal circuitry to interpret sensory information and generate related neuromuscular responses. This approach enables the spinal cord to generate lower limb extension patterns during weight bearing, allowing individuals with SCI to achieve upright standing. We have shown that the human spinal cord can generate some standing postural responses during self-initiated body weight shifting. In this study, we investigated the ability of individuals with motor complete SCI receiving epidural stimulation to generate standing reactive postural responses after external perturbations were applied at the trunk. A cable-driven robotic device was used to provide constant assistance for pelvic control and to deliver precise trunk perturbations while participants used their hands to grasp onto handlebars for self-balance support (hands-on) as well as when participants were without support (free-hands). Five individuals with motor complete SCI receiving lumbosacral spinal cord epidural stimulation parameters specific for standing (Stand-scES) participated in this study. Trunk perturbations (average magnitude: 17 ± 3% body weight) were delivered randomly in the four cardinal directions. Participants attempted to control each perturbation such that upright standing was maintained and no additional external assistance was needed. Lower limb postural responses were generally more frequent, larger in magnitude, and appropriately modulated during the free-hands condition. This was associated with trunk displacement and lower limb loading modulation that were larger in the free-hands condition. Further, we observed discernible lower limb muscle synergies that were similar between the two perturbed standing conditions. These findings suggest that the human spinal circuitry involved in postural control retains the ability to generate meaningful lower limb postural responses after SCI when its excitability is properly modulated. Moreover, lower limb postural responses appear enhanced by a standing environment without upper limb stabilization that promotes afferent inputs associated with a larger modulation of ground reaction forces and trunk kinematics. These findings should be considered when developing future experimental frameworks aimed at studying upright postural control and activity-based recovery training protocols aimed at promoting neural plasticity and sensory-motor recovery.

Precise cortical contributions to sensorimotor feedback control during reactive balance

The role of the cortex in shaping automatic whole-body motor behaviors such as walking and balance is poorly understood. Gait and balance are typically mediated through subcortical circuits, with the cortex becoming engaged as needed on an individual basis by task difficulty and complexity. However, we lack a mechanistic understanding of how increased cortical contribution to whole-body movements shapes motor output. Here we use reactive balance recovery as a paradigm to identify relationships between hierarchical control mechanisms and their engagement across balance tasks of increasing difficulty in young adults. We hypothesize that parallel sensorimotor feedback loops engaging subcortical and cortical circuits contribute to balance-correcting muscle activity, and that the involvement of cortical circuits increases with balance challenge. We decomposed balance-correcting muscle activity based on hypothesized subcortically- and cortically-mediated feedback components driven by similar sensory information, but with different loop delays. The initial balance-correcting muscle activity was engaged at all levels of balance difficulty. Its onset latency was consistent with subcortical sensorimotor loops observed in the lower limb. An even later, presumed, cortically-mediated burst of muscle activity became additionally engaged as balance task difficulty increased, at latencies consistent with longer transcortical sensorimotor loops. We further demonstrate that evoked cortical activity in central midline areas measured using electroencephalography (EEG) can be explained by a similar sensory transformation as muscle activity but at a delay consistent with its role in a transcortical loop driving later cortical contributions to balance-correcting muscle activity. These results demonstrate that a neuromechanical model of muscle activity can be used to infer cortical contributions to muscle activity without recording brain activity. Our model may provide a useful framework for evaluating changes in cortical contributions to balance that are associated with falls in older adults and in neurological disorders such as Parkinson's disease.

Generalizability of foot-placement control strategies during unperturbed and perturbed gait

Control of foot placement is an essential strategy for maintaining balance during walking. During unperturbed, steady-state walking, foot placement can be accurately described as a linear function of the body's center of mass state at midstance. However, it is uncertain if this mapping from center of mass state to foot placement generalizes to larger perturbations that may be more likely to cause falls. These perturbations may cause balance disturbances and generate reactive control strategies not observed during unperturbed walking. Here, we used unpredictable changes in treadmill speed to assess the generalizability of foot placement mappings identified during unperturbed walking. We found that foot placement mappings generalized poorly from unperturbed to perturbed walking and differed for forward versus backward perturbations. We also used singular value decomposition of the mapping matrix to reveal that people were more sensitive to backward versus forward perturbations. Together, these results indicate that control of foot placement during losses of balance differs from the control strategies used during unperturbed walking. Better characterization of human balance control strategies could improve our understanding of why different neuromotor disorders result in heightened fall risk and inform the design of controllers for balance-assisting devices.

Passive Dynamics of the Head, Neck and Forelimb in Equine Foetuses-An Observational Study

Passive dynamics is an aspect of locomotion which is entirely dependent on the mechanical configuration and linkages of adjacent body segments. Tension distribution along mechanical linkages enables the execution of movement patterns with reduced need for complex neurological pathways and may play a role in reestablishing postural stability following external disturbances. Here we demonstrate a uni-directional mechanical relationship between the equine forelimb, head and neck, which may have implications for balance and forelimb loading in the horse. These observations suggest that forelimb, head and neck movement coordination (observed in the horse during unrestrained locomotion) is significantly influenced by the mechanical linkages between body segments, rather than being entirely dependent on neurological input as previously thought. This highlights the potential significance of research directed at investigating passively induced movements in understanding common locomotory patterns. Additionally, it suggests a mode of postural control which may provide instantaneous adjustments to postural disturbances, thus promoting rapid and efficient locomotion.

Mobile Brain Imaging to Examine Task-Related Cortical Correlates of Reactive Balance: A Systematic Review

This systematic review examined available findings on spatial and temporal characteristics of cortical activity in response to unpredicted mechanical perturbations. Secondly, this review investigated associations between cortical activity and behavioral/biomechanical measures. Databases were searched from 1980-2021 and a total of 35 cross-sectional studies (31 EEG and 4 fNIRS) were included. Majority of EEG studies assessed perturbation-evoked potentials (PEPs), whereas other studies assessed changes in cortical frequencies. Further, fNIRS studies assessed hemodynamic changes. The PEP-N1, commonly identified at sensorimotor areas, was most examined and was influenced by context prediction, perturbation magnitude, motor adaptation and age. Other PEPs were identified at frontal, parietal and sensorimotor areas and were influenced by task position. Further, changes in cortical frequencies were observed at prefrontal, sensorimotor and parietal areas and were influenced by task difficulty. Lastly, hemodynamic changes were observed at prefrontal and frontal areas and were influenced by task prediction. Limited studies reported associations between cortical and behavioral outcomes. This review provided evidence regarding the involvement of cerebral cortex for sensory processing of unpredicted perturbations, error-detection of expected versus actual postural state, and planning and execution of compensatory stepping responses. There is still limited evidence examining cortical activity during reactive balance tasks in populations with high fall-risk.

Prospective comparative study between knee alignment-oriented static and dynamic balance exercise in patellofemoral pain syndrome patients with dynamic knee valgus

Exercise therapy has been reported as an effective treatment method for patellofemoral pain syndrome (PFPS). However, there is a lack of studies regarding the effectiveness of balance exercise in the treatment of patients with PFPS. This study aimed to prospectively compare changes in proprioception, neuromuscular control, knee muscle strength, and patient-reported outcomes between patients with PFPS treated with knee alignment-oriented static balance exercise (SBE) and dynamic balance exercise (DBE). The participants were divided into 2 groups: 17 knee alignment-oriented SBE group and 19 knee alignment-oriented DBE group. Proprioception was assessed by dynamic postural stability using postural stabilometry. Neuromuscular control and knee muscle strength were measured for acceleration time and peak torque in quadriceps muscle using an isokinetic device. Patient-reported outcomes were evaluated using a visual analog scale for pain and the Kujala Anterior Knee Pain Scale. There was greater improvement in dynamic postural stability (0.9 ± 0.3 vs 1.2 ± 0.5; 95% confidence interval [CI]: 0, 0.6; Effect size: 0.72; P = .021) and quadriceps AT (40.5 ± 14.3 vs 54.1 ± 16.9; 95% CI: 2.9, 24.2; Effect size: 0.86; P = .014) in the DBE group compared to the SBE group. Knee alignment-oriented DBE can be more effective in improving dynamic postural stability and quadriceps muscle reaction time compared with the knee alignment-oriented SBE in PFPS patients with dynamic knee valgus.

Postural displacement induced by electrical stimulation; A new approach to examine postural recovery

Background

Controlling upright posture entails acute adjustments by the neuromuscular system to keep the center of mass (COM) within the limits of a relatively small base of support. Sudden displacement of the COM triggers several strategies and balance recovery mechanisms to prevent excessive COM displacement.

New method

We have examined and quantified a new approach to induce an internal neuromuscular perturbation in standing posture on 15 healthy individuals to provide an insight into the mechanism of loss of balance (LOB). The method comprises eliciting an H-reflex protocol while subjects are standing which produces a contraction in soleus and gastrocnemius muscles. We have also defined analytical techniques to provide biomarkers of balance control during perturbation. We used M-Max unilaterally or bilaterally and induced a forward or sideway perturbation. The vector analysis and the Equilibrium Point calculations defined here can quantify the amplitude, direction, and evolution of the perturbation.

Results

Clear patterns of loss of balance due to stimulation was observed. Compared to quiet standing, the density of the EPs substantially increased in the perturbation phase. Leftward stimulation produced significantly higher number of EPs compared to the bilateral stimulation condition which could be due to the fact that the left leg was the nondominant side in all our subjects.

Comparison and conclusion

In this study we provide a proof-of-concept technique for examining recovery from perturbation. The advantage of this technique is that it provides a safe perturbation, is internally induced at the spinal cord level, and is free from other factors that might complicate the recovery analysis (e.g., locomotion and the integration of the spinal pattern generator and cutaneous pathways in mediating changes). We have shown that the perturbation induced by this method can be quantified as vectors. We have also shown that the density of instantaneous equilibrium points (EPs) could be a good biomarker for defining and examining the perturbation phase. Thus, this protocol and analysis provides a unique individual assessment of recovery which can be used to assess interventions. Finally, given that the maximal motor response is used as the perturbation (e.g., M-max) it is highly reliable and reproducible within an individual patient.

Non-invasive Brain Stimulation for Chronic Pain: State of the Art and Future Directions

As a technique that can guide brain plasticity, non-invasive brain stimulation (NIBS) has the potential to improve the treatment of chronic pain (CP) because it can interfere with ongoing brain neural activity to regulate specific neural networks related to pain management. Treatments of CP with various forms of NIBS, such as repetitive transcranial magnetic stimulation (rTMS) and transcranial direct current stimulation (tDCS), using new parameters of stimulation have achieved encouraging results. Evidence of moderate quality indicates that high-frequency rTMS of the primary motor cortex has a clear effect on neuropathic pain (NP) and fibromyalgia. However, evidence on its effectiveness regarding pain relief in other CP conditions is conflicting. Concerning tDCS, evidence of low quality supports its benefit for CP treatment. However, evidence suggesting that it exerts a small treatment effect on NP and headaches is also conflicting. In this paper, we describe the underlying principles behind these commonly used stimulation techniques; and summarize the results of randomized controlled trials, systematic reviews, and meta-analyses. Future research should focus on a better evaluation of the short-term and long-term effectiveness of all NIBS techniques and whether they decrease healthcare use, as well as on the refinement of selection criteria.

Effects of Occlusal Splints on Spinal Posture in Patients with Temporomandibular Disorders: A Systematic Review

There is still a gap in the scientific knowledge on the linkage between craniofacial structure and spinal postural control in temporomandibular disorder (TMD) patients. This systematic review aimed to assess the role of occlusal splints on spinal posture of TMD patients. PubMed, Web of Science, and Scopus were systematically searched from inception until 5 January 2022 to identify observational studies with a longitudinal study design presenting: patients with diagnosis of TMD according to the Diagnostic Criteria for Temporomandibular Disorders (DC/TMD); occlusal splint therapy as intervention; postural assessment as outcome. Out of 133 records identified, 104 were suitable for data screening, and only 7 articles were included satisfying the eligibility criteria. We found that occlusal splints might have a positive effect on posture in TMD patients, albeit there is little evidence of appropriate investigation for postural assessment. This systematic review suggested that the occlusal splint might be considered a non-invasive therapeutic approach for patients with TMD. However, the low number of studies with high-quality methodology in these patients showed an urgent need for further research using combined force platform stabilometry and kinematic evaluation of the spine to investigate the impact of occlusal splints on posture.

Simple decentralized control mechanism that enables limb adjustment for adaptive quadruped running

Quadrupeds exhibit versatile and adaptive running by exploiting the flying phase during the stride cycle. Various interlimb coordination mechanisms focusing on mechanical loads during the stance phase have been proposed to understand the underlying control mechanism, and various gait patterns have been reproduced. However, the essential control mechanism required to achieve both steady running patterns and non-steady behaviours, such as jumping and landing, remains unclear. Therefore, we focus on the vertical motions of the body parts and propose a new decentralized interlimb coordination mechanism. The simulation results demonstrate that the robot can generate efficient and various running patterns in response to the morphology of the body. Furthermore, the proposed model allows the robot to smoothly change its behaviour between steady running and non-steady landing depending on the situation. These results suggest that the steady and non-steady behaviours in quadruped adaptive running may share a common simple control mechanism based on the mechanical loads and vertical velocities of the body parts.

Postural sway in the moving room scenario: New evidence for functional dissociation between self-motion perception and postural control

Postural control in quiet standing is often explained by a reflexive response to optical flow, the apparent motion of environmental objects in a visual scene. However, moving room experiments show that even small-amplitude body sway can evoke odd sensations or motion sickness, indicating that a consciousness factor may also be involved. Studies targeting perception of self-motion, vection, typically use rapid visual stimuli moving in a single direction to maintain a constant feeling of vection, and there are few studies of vection using low-speed sinusoidal visual stimuli similar to human pendular movement. In the present study we searched for changes in postural control during periods of vection during quiet standing. Participants (N = 19, age = 20.4 ±1.1 years) were shown dynamic visual stimuli in the form of sinusoidally expanding and contracting random dots, and the stimuli speed and visual field were manipulated. Posture was continually evaluated using Center of Pressure (CoP) measurements. Participants were also asked to report feelings of vection, both by pressing a button during the trial and through an overall rating at the end of each trial. Using repeated-measures ANOVA, we assessed changes in the CoP and vection variables between experimental conditions, as well as possible interactions between the variables. The results show that postural reaction and vection were both affected by the visual stimuli and varied with speed. The peripheral visual field was found to couple to stronger feeling of vection and better quality of postural control. However, no significant relationship between postural control and vection, nor evidence of vection interaction to the relationship between optical flow and postural control, was found. Based on our results we conclude that for postural stability during quiet standing, visual cues dominate over any potential consciousness factor arising due to vection.

Spectral responses across a dorsal-ventral array of dermal sensilla in the medicinal leech

How do animals use visual systems to extract specific features of a visual scene and respond appropriately? The medicinal leech, Hirudo verbana, is a predatory, quasi-amphibious annelid with a rich sensorium that is an excellent system in which to study how sensory cues are encoded, and how key features of visual images are mapped into the CNS. The leech visual system is broadly distributed over its entire body, consisting of five pairs of cephalic eyecups and seven segmentally iterated pairs of dermal sensilla in each mid-body segment. Leeches have been shown to respond behaviorally to both green and near ultraviolet light (UV, 365–375 nm). Here, we used electrophysiological techniques to show that spectral responses by dermal sensilla are mapped across the dorsal–ventral axis, such that the ventral sensilla respond strongly to UV light, while dorsal sensilla respond strongly to visible light, broadly tuned around green. These results establish how key features of visual information are initially encoded by spatial mapping of photo-response profiles of primary photoreceptors and provide insight into how these streams of information are presented to the CNS to inform behavioral responses.

Breaking a dogma: acute anti-inflammatory treatment alters both post-lesional functional recovery and endogenous adaptive plasticity mechanisms in a rodent model of acute peripheral vestibulopathy

BACKGROUND

Due to their anti-inflammatory action, corticosteroids are the reference treatment for brain injuries and many inflammatory diseases. However, the benefits of acute corticotherapy are now being questioned, particularly in the case of acute peripheral vestibulopathies (APV), characterized by a vestibular syndrome composed of sustained spinning vertigo, spontaneous ocular nystagmus and oscillopsia, perceptual-cognitive, posturo-locomotor, and vegetative disorders. We assessed the effectiveness of acute corticotherapy, and the functional role of acute inflammation observed after sudden unilateral vestibular loss.

METHODS

We used the rodent model of unilateral vestibular neurectomy, mimicking the syndrome observed in patients with APV. We treated the animals during the acute phase of the vestibular syndrome, either with placebo or methylprednisolone, an anti-inflammatory corticosteroid. At the cellular level, impacts of methylprednisolone on endogenous plasticity mechanisms were assessed through analysis of cell proliferation and survival, glial reactions, neuron's membrane excitability, and stress marker. At the behavioral level, vestibular and posturo-locomotor functions' recovery were assessed with appropriate qualitative and quantitative evaluations.

RESULTS

We observed that acute treatment with methylprednisolone significantly decreases glial reactions, cell proliferation and survival. In addition, stress and excitability markers were significantly impacted by the treatment. Besides, vestibular syndrome's intensity was enhanced, and vestibular compensation delayed under acute methylprednisolone treatment.

CONCLUSIONS

We show here, for the first time, that acute anti-inflammatory treatment alters the expression of the adaptive plasticity mechanisms in the deafferented vestibular nuclei and generates enhanced and prolonged vestibular and postural deficits. These results strongly suggest a beneficial role for acute endogenous neuroinflammation in vestibular compensation. They open the way to a change in dogma for the treatment and therapeutic management of vestibular patients.

Cortical Beta Oscillatory Activity Evoked during Reactive Balance Recovery Scales with Perturbation Difficulty and Individual Balance Ability

Cortical beta oscillations (13-30 Hz) reflect sensorimotor processing, but are not well understood in balance recovery. We hypothesized that sensorimotor cortical activity would increase under challenging balance conditions. We predicted greater beta power when balance was challenged, either by more difficult perturbations or by lower balance ability. In 19 young adults, we measured beta power over motor cortical areas (electroencephalography, Cz electrode) during three magnitudes of backward support -surface translations. Peak beta power was measured during early (50-150 ms), late (150-250 ms), and overall (0-400 ms) time bins, and wavelet-based analyses quantified the time course of evoked beta power. An ANOVA was used to compare peak beta power across perturbation magnitudes in each time bin. We further tested the association between perturbation-evoked beta power and individual balance ability measured in a challenging beam walking task. Beta power increased ~50 ms after perturbation, and to a greater extent in larger perturbations. Lower individual balance ability was associated with greater beta power in only the late (150-250 ms) time bin. These findings demonstrate greater sensorimotor cortical engagement under more challenging balance conditions, which may provide a biomarker for reduced automaticity in balance control that could be used in populations with neurological impairments.

Targeted Walking in Incomplete Spinal Cord Injury: Role of Corticospinal Control

Locomotor recovery after incomplete spinal cord injury (iSCI) is influenced by spinal and supraspinal networks. Conventional clinical gait analysis fails to differentiate between these components. There is evidence that corticospinal control is enhanced during targeted walking, where each foot must be continuously placed on visual targets in randomized order. This study investigates the potential of targeted walking in the functional assessment of corticospinal integrity. Twenty-one controls and 16 individuals with chronic iSCI performed normal and targeted walking on a treadmill while electromyograms (EMGs) and kinematics were recorded. Precision (% of accurate foot placements) in targeted walking was significantly lower in individuals with iSCI (82.9 ± 14.7%, controls: 94.9 ± 4.0%). Although the overall kinematic pattern was comparable between walking conditions, controls showed significantly higher semitendinosus (ST) activity before heel-strike during targeted walking. This was accompanied by a shift of relative EMG intensity from 90-120 Hz to lower frequencies of 20-60 Hz, previously associated with corticospinal control of muscle activity. Targeted walking in individuals with iSCI evoked smaller EMG changes, suggesting that the switch to more corticospinal control is impaired. Accordingly, mildly impaired iSCI individuals revealed higher adaptations to the targeted walking task than more-impaired individuals. Recording of EMGs during targeted walking holds potential as a research tool to reveal further insights into the neuromuscular control of locomotion. It also complements findings of pre-clinical studies and is a promising novel surrogate marker of integrity of corticospinal control in individuals with iSCI and other neurological impairments. Future studies should investigate its potential for diagnosis or tracking recovery during rehabilitation.

Acute effects of kinesiology tape tension on soleus muscle h-reflex modulations during lying and standing postures

Kinesiology tape (KT) has been widely used in the areas of sports and rehabilitation. However, there is no gold standard for the tape tension used during a KT application. The purpose of this study was to examine the effects of KT application with different tension intensities on soleus muscle Hoffmann-reflex (H-reflex) modulation during lying and standing postures. Fifteen healthy university students were tested with 3 tape tension intensities during separate visits with a randomized sequence: tape-on no tension (0KT), moderate (about 50% of the maximal tape tension: (ModKT), and maximal tape tension (MaxKT). During each experimental visit, the H-reflex measurements on the soleus muscle were taken before, during, and after the KT application for both lying and standing postures. The H-wave and M-wave recruitment curves were generated using surface electromyography (EMG). There was a main effect for posture (p = 0.001) for the maximal peak-to-peak amplitude of the H-wave and M-wave (H_max/M_max) ratio, showing the depressed H_max/M_max ratio during standing, when compared to the lying posture. Even though the tension factor had a large effect (η_p² = 0.165), different tape tensions showed no significant differential effects for the H_max/M_max ratio. The spinal motoneuron excitability was not altered, even during the maximal tension KT application on the soleus muscle. Thus, the tension used during a KT application should not be a concern in terms of modulating the sensorimotor activity ascribed to elastic taping during lying and standing postures.

Identification of New Biomarkers of Posturo-Locomotor Instability in a Rodent Model of Vestibular Pathology

The vestibular system plays a crucial role in maintaining postural balance. Unilateral vestibular lesions result in a typical syndrome characterized by postural imbalance, altered locomotor patterns and gaze stabilization, as well as cognitive and neurovegetative disorders. One of the main difficulties encountered in the development of new anti-vertigo drugs is the lack of sensitivity in the evaluation of this syndrome. Qualitative assessments of the vestibular syndrome have been developed, but methods of conducting quantitative evaluations are critically lacking. Recently, assessments with a dynamic weight-bearing device (DWB®, Bioseb) revealed postural alterations in rats subjected to unilateral vestibular neurectomy (UVN). Our team is evaluating a new version of this device capable of quantifying additional parameters of postural and locomotor equilibrium. The objective of this study was to use this device to assess these new posturo-locomotor parameters in a rat model of a vestibular pathology. The biomarkers measured by this device are as follows: the barycenter, the support surface and the weight distribution of the rats when they were moving or stationary. Before UVN, the rats showed a symmetric distribution of their weight along the lateral axis. In the acute phase after UVN on the left side, the rats distributed more weight on the right side than on the left side and then distributed more weight on the left side. These results corroborate those presented in our previous study. The support surface of the rats increased between 1 day and 30 days after UVN, and the barycenter distribution reflected the weight distribution. In addition, our results show smaller changes in the weight distributions when the animals are moving compared with when they are stationary in the acute phase after UVN. This study provides new information on the static and dynamic postural balance patterns observed after unilateral vestibular loss in rats. These data are relevant because they objectively quantify the posturo-locomotor component of vestibular syndrome as well as the compensatory strategies used after vestibular loss. These results may guide the development of rehabilitation protocols for vestibular patients and the validation of pharmacological compounds favoring the restoration of equilibrium.

Visual Feedback and Postural Control in Multiple Sclerosis

As people with multiple sclerosis (pwMS) manifest heterogeneous demyelinating lesions that could affect somatosensory or vestibular ways, visual stimulus as feedback could be especially relevant to achieve postural control. This has clinical importance for the development of preventive measures and rehabilitation therapies in order to avoid falls and accidents in this group. In our study, we objectively evaluated the influence of visual feedback on the stabilization of balance in pwMS versus healthy controls (HC) and its potential utility in clinical evaluation. Static posturography tests were performed in 99 pwMS and 30 HC. Subjects stood on a force platform with open and closed eyes. During this procedure, three balance parameters were obtained for both vision conditions: average sway, average speed, and average speed of sway. Neurostatus-Expanded Disease Disability Score (EDSS) and Multiple Sclerosis Functional Composite (MSFC) were performed in parallel as well. A two-way mixed repeated measures ANCOVA, controlling for sex and age, was performed to evaluate the effect of vision, MS diagnosis, and the interaction of both in static posturography parameters. The difference between both closed and open eyes conditions was calculated for each parameter and further analyzed according to MS-relevant clinical variables. The magnitude of the vision effect differed between pwMS and HC as a significant interaction between the vision and the MS diagnosis in the delineated area (p < 0.001) and average speed of sway (p = 0.001) was seen. These parameters had a greater increase in pwMS than in HC after closing eyes. For the average sway, a significant main effect of vision was present (p = 0.047). Additionally, the differences obtained between open and closed eyes conditions assessed with the delineated area and average speed of sway were moderately correlated to the assessed clinical tests EDSS (r = 0.405 and r = 0.329, respectively) and the MSFC (r = −0.385 and r = −0.259, respectively). In our study, pwMS were more dependent of visual feedback than HC to maintain postural control. This easy and short evaluation by static posturography could support the development of targeted preventive measures and interventions in pwMS.

Ancestral persistence of vestibulospinal reflexes in axial muscles in humans

Most studies addressing the role of vestibulospinal reflexes in balance maintenance have mainly focused on responses in the lower limbs, while limited attention has been paid to the output in trunk and back muscles. To address this issue, we tested whether electromyographic (EMG) responses to galvanic vestibular stimulations (GVS) were modulated similarly in back and leg muscles, in situations where the leg muscle responses to GVS are known to be attenuated. Body sway and surface EMG signals were recorded in the paraspinal and limb muscles of humans (n = 19) under three complementary conditions. During treadmill locomotion, EMG responses in the lower limbs were observed only during stance, whereas responses in trunk muscles were observed during all phases of the locomotor cycle. During upright standing, a slight head contact abolished the responses in the lower limbs, while the responses remained present in back muscles. Similarly, during parabolic flight-induced microgravity, EMG responses in lower limb muscles were suppressed but remained in axial muscles despite the abolished gravitational otolithic drive. Our results suggest a differentiated control of axial and appendicular muscles when a perturbation is detected by vestibular inputs. The persistence and low modulation of axial muscle responses suggests that a hard-wired reflex is functionally efficient to maintain posture. By contrast, the ankle responses to GVS occur only in balance tasks when proprioceptive feedback is congruent. This study using GVS in microgravity is the first to present an approach delineating feedforward vestibular control in unconstrained environment.NEW & NOTEWORTHY This study addresses the extent of conservation of trunk muscle control in humans. Results show that galvanic vestibular stimulation-evoked vestibular responses in trunk muscles remain strong in conditions where leg muscle responses are downmodulated (walking, standing, microgravity). This suggests a phylogenetically conserved blueprint of sensorimotor organization, with strongly hardwired vestibulospinal inputs to axial motoneurons and a higher degree of flexibility in the later emerging limb control system.

Effect of Repetitive Transcranial Magnetic Stimulation on Pain Management: A Systematic Narrative Review

Recently, clinicians have been using repetitive transcranial magnetic stimulation (rTMS) for treating various pain conditions. This systematic narrative review aimed to examine the use and efficacy of rTMS for controlling various pain conditions. A PubMed search was conducted for articles that were published until June 7, 2019 and used rTMS for pain alleviation. The key search phrase for identifying potentially relevant articles was (repetitive transcranial magnetic stimulation AND pain). The following inclusion criteria were applied for article selection: (1) patients with pain, (2) rTMS was applied for pain management, and (3) follow-up evaluations were performed after rTMS stimulation to assess the reduction in pain. Review articles were excluded. Overall, 1,030 potentially relevant articles were identified. After reading the titles and abstracts and assessing eligibility based on the full-text articles, 106 publications were finally included in our analysis. Overall, our findings suggested that rTMS is beneficial for treating neuropathic pain of various origins, such as central pain, pain from peripheral nerve disorders, fibromyalgia, and migraine. Although data on the use of rTMS for orofacial pain, including trigeminal neuralgia, phantom pain, low back pain, myofascial pain syndrome, pelvic pain, and complex regional pain syndrome, were promising, there was insufficient evidence to determine the efficacy of rTMS for treating these conditions. Therefore, further studies are needed to validate the effects of rTMS on pain relief in these conditions. Overall, this review will help guide clinicians in making informed decisions regarding whether rTMS is an appropriate option for managing various pain conditions.

Current Principles of Motor Control, with Special Reference to Vertebrate Locomotion

The vertebrate control of locomotion involves all levels of the nervous system from cortex to the spinal cord. Here, we aim to cover all main aspects of this complex behavior, from the operation of the microcircuits in the spinal cord to the systems and behavioral levels and extend from mammalian locomotion to the basic undulatory movements of lamprey and fish. The cellular basis of propulsion represents the core of the control system, and it involves the spinal central pattern generator networks (CPGs) controlling the timing of different muscles, the sensory compensation for perturbations, and the brain stem command systems controlling the level of activity of the CPGs and the speed of locomotion. The forebrain and in particular the basal ganglia are involved in determining which motor programs should be recruited at a given point of time and can both initiate and stop locomotor activity. The propulsive control system needs to be integrated with the postural control system to maintain body orientation. Moreover, the locomotor movements need to be steered so that the subject approaches the goal of the locomotor episode, or avoids colliding with elements in the environment or simply escapes at high speed. These different aspects will all be covered in the review.

Postural control in healthy adults: Determinants of trunk sway assessed with a chest-worn accelerometer in 12 quiet standing tasks

Many diseases and conditions decrease the ability to control balance. In clinical settings, there is therefore a major interest in the assessment of postural control. Trunk accelerometry is an easy, low-cost method used for balance testing and constitutes an alternative method to the posturography using force platforms. The objective was to assess the responsiveness of accelerometry in a battery of 12 quiet standing tasks. We evaluated the balance of 100 healthy adults with an accelerometer fixed onto the sternum. We used the average amplitude of acceleration as an indirect measure of postural sways. The tasks of increased difficulty were realized with or without vision. The battery of tasks was repeated four times on two different days to assess reliability. We analyzed the extent to which the task difficulty and the absence of vision affected the trunk sway. The influence of individual characteristics (age, height, mass, sex, and physical activity level) was also assessed. The reliability analysis revealed that four repetitions of the battery of tasks are needed to reach a high accuracy level (mean ICC = 0.85). The results showed that task difficulty had a very large effect on trunk sways and that the removal of vision further increased sways. Concerning the effects of individual characteristics, we observed that women tended to oscillate more than men did in tasks of low difficulty. Age and physical activity level also had significant effects, whereas height and mass did not. In conclusion, age, sex, and physical fitness are confounders that should be considered when assessing patients’ balance. A battery of simple postural tasks measured by upper-trunk accelerometry can be a useful method for simple balance evaluation in clinical settings.

Vestibular and corticospinal control of human body orientation in the gravitational field

Body orientation with respect to the direction of gravity changes when we lean forward from upright standing. We tested the hypothesis that during upright standing, the nervous system specifies the referent body orientation that defines spatial thresholds for activation of multiple muscles across the body. To intentionally lean the body forward, the system is postulated to transfer balance and stability to the leaned position by monotonically tilting the referent orientation, thus increasing the activation thresholds of ankle extensors and decreasing their activity. Consequently, the unbalanced gravitational torque would start to lean the body forward. With restretching, ankle extensors would be reactivated and generate increasing electromyographic (EMG) activity until the enhanced gravitational torque would be balanced at a new posture. As predicted, vestibular influences on motoneurons of ankle extensors evaluated by galvanic vestibular stimulation were smaller in the leaned compared with the upright position, despite higher tonic EMG activity. Defacilitation of vestibular influences was also observed during forward leaning when the EMG levels in the upright and leaned position were equalized by compensating the gravitational torque with a load. The vestibular system is involved in the active control of body orientation without directly specifying the motor outcome. Corticospinal influences originating from the primary motor cortex evaluated by transcranial magnetic stimulation remained similar at the two body postures. Thus, in contrast to the vestibular system, the corticospinal system maintains a similar descending facilitation of motoneurons of leg muscles at different body orientations. The study advances the understanding of how body orientation is controlled. NEW & NOTEWORTHY The brain changes the referent body orientation with respect to gravity to lean the body forward. Physiologically, this is achieved by shifts in spatial thresholds for activation of ankle muscles, which involves the vestibular system. Results advance the understanding of how the brain controls body orientation in the gravitational field. The study also extends previous evidence of empirical control of motor function, i.e., without the reliance on model-based computations and direct specification of motor outcome.

Referent control of the orientation of posture and movement in the gravitational field

This study addresses the question of how posture and movement are oriented with respect to the direction of gravity. It is suggested that neural control levels coordinate spatial thresholds at which multiple muscles begin to be activated to specify a referent body orientation (RO) at which muscle activity is minimized. Under the influence of gravity, the body is deflected from the RO to an actual orientation (AO) until the emerging muscle activity and forces begin to balance gravitational forces and maintain body stability. We assumed that (1) during quiet standing on differently tilted surfaces, the same RO and thus AO can be maintained by adjusting activation thresholds of ankle muscles according to the surface tilt angle; (2) intentional forward body leaning results from monotonic ramp-and-hold shifts in the RO; (3) rhythmic oscillation of the RO about the ankle joints during standing results in body swaying. At certain sway phases, the AO and RO may transiently overlap, resulting in minima in the activity of multiple muscles across the body. EMG kinematic patterns of the 3 tasks were recorded and explained based on the RO concept that implies that these patterns emerge due to referent control without being pre-programmed. We also confirmed the predicted occurrence of minima in the activity of multiple muscles at specific body configurations during swaying. Results re-affirm previous rejections of model-based computational theories of motor control. The role of different descending systems in the referent control of posture and movement in the gravitational field is considered.

Cerebral Dynamics during the Observation of Point-Light Displays Depicting Postural Adjustments

Objective: As highly social creatures, human beings rely part of their skills of identifying, interpreting, and predicting the actions of others on the ability of perceiving biological motion. In the present study, we aim to investigate the electroencephalographic (EEG) cerebral dynamics involved in the coding of postural control and examine whether upright stance would be codified through the activation of the temporal-parietal cortical network classically enrolled in the coding of biological motion. Design: We registered the EEG activity of 12 volunteers while they passively watched point light displays (PLD) depicting quiet stable (QB) and an unstable (UB) postural situations and their respective scrambled controls (QS and US). In a pretest, 13 volunteers evaluated the level of stability of our two biological stimuli through a stability scale. Results: Contrasting QB vs. QS revealed a typical ERP difference in the right temporal-parietal region at an early 200-300 ms time window. Furthermore, when contrasting the two biological postural conditions, UB vs. QB, we found a higher positivity in the 400-600 ms time window for the UB condition in central-parietal electrodes, lateralized to the right hemisphere. Conclusions: These results suggest that PLDs depicting postural adjustments are coded in the brain as biological motion, and that their viewing recruit similar networks with those engaged in postural stability control. Additionally, higher order cognitive processes appear to be engaged in the identification of the postural instability level. Disentangling the EEG dynamics during the observation of postural adjustments could be very useful for further understanding the neural mechanisms underlying postural control.

Activity Monitoring and Heart Rate Variability as Indicators of Fall Risk: Proof-of-Concept for Application of Wearable Sensors in the Acute Care Setting

Growing concern for falls in acute care settings could be addressed with objective evaluation of fall risk. The current proof-of-concept study evaluated the feasibility of using a chest-worn sensor during hospitalization to determine fall risk. Physical activity and heart rate variability (HRV) of 31 volunteers admitted to a 29-bed adult inpatient unit were recorded using a single chest-worn sensor. Sensor data during the first 24-hour recording were analyzed. Participants were stratified using the Hendrich II fall risk assessment into high and low fall risk groups. Univariate analysis revealed age, daytime activity, nighttime side lying posture, and HRV were significantly different between groups. Results suggest feasibility of wearable technology to consciously monitor physical activity, sleep postures, and HRV as potential markers of fall risk in the acute care setting. Further study is warranted to confirm the results and examine the efficacy of the proposed wearable technology to manage falls in hospitals. [Journal of Gerontological Nursing, 43(7), 53-62.].

Effects of bicuculline application on the somatosensory responses of secondary vestibular neurons

Limb somatosensory signals modify the discharge of vestibular neurons and elicit postural reflexes, which stabilize the body position. The aim of this study was to investigate the contribution of the γ-amino-butyric-acid (GABA) to the responsiveness of vestibular neurons to somatosensory inputs. The activity of 128 vestibular units was recorded in anesthetized rats in resting conditions and during sinusoidal foreleg rotation around the elbow or shoulder joints (0.026-0.625Hz, 45° peak amplitude). None of the recorded units was influenced by elbow rotation, while 40% of them responded to shoulder rotation. The selective GABAA antagonist receptor, bicuculline methiodine (BIC), was applied by microiontophoresis on single vestibular neurons and the changes in their activity at rest and during somatosensory stimulation was studied. In about half of cells the resting activity increased after the BIC application: 75% of these neurons showed also an increased response to somatosensory inputs whereas 17% exhibited a decrease. Changes in responsiveness in both directions were detected also in the units whose resting activity was not influenced by BIC. These data suggest that the responses of vestibular neurons to somatosensory inputs are modulated by GABA through a tonic release, which modifies the membrane response to the synaptic current. It is also possible that a phasic release of GABA occurs during foreleg rotation, shaping the stimulus-elicited current passing through the membrane. If this is the case, the changes in the relative position of body segments would modify the GABA release inducing changes in the vestibular reflexes and in learning processes that modify their spatio-temporal development.

Bumetanide, an Inhibitor of NKCC1 (Na-K-2Cl Cotransporter Isoform 1), Enhances Propofol-Induced Loss of Righting Reflex but Not Its Immobilizing Actions in Neonatal Rats

Gamma-aminobutyric acid (GABA) has been shown to induce excitation on immature neurons due to increased expression of Na+-K+-2Cl- co-transporter isoform 1 (NKCC1), and the transition of GABAergic signaling from excitatory to inhibitory occurs before birth in the rat spinal cord and spreads rostrally according to the developmental changes in cation-chloride co-transporter expression. We previously showed that midazolam activates the hippocampal CA3 area and induces less sedation in neonatal rats compared with adolescent rats in an NKCC1-dependent manner. In the present study, we tested the hypothesis that propofol-induced loss of righting reflex (LORR) but not immobilizing actions are modulated by NKCC1-dependent mechanisms and reduced in neonatal rats compared with adolescent rats. We estimated neuronal activity in the cortex, hippocampus and thalamus after propofol administration with or without bumetanide, an NKCC1 inhibitor, by immunostaining of phosphorylated cyclic adenosine monophosphate-response element binding protein (pCREB). We studied effects of bumetanide on propofol-induced LORR and immobilizing actions in postnatal day 7 and 28 (P7 and P28) rats. The pCREB expression in the cortex (P = 0.001) and hippocampus (P = 0.01) was significantly greater in the rats receiving propofol only than in the rats receiving propofol plus bumetanide at P 7. Propofol-induced LORR or immobilizing effects did not differ significantly between P7 and P28. Bumetanide significantly enhanced propofol-induced LORR (P = 0.031) but not immobilization in P7 rats. These results are partially consistent with our hypothesis. They suggest that propofol may activate the rostral but not caudal central nervous system dependently on NKCC1, and these differential actions may underlie the different properties of sedative and immobilizing actions observed in neonatal rats.

Characteristics of Autocorrelation Structure of Lower Extremity Functional Laterality in Disturbed and Undisturbed Bipedal Upright Stance

Purpose. It is posited that functional laterality is influenced by the generation and conduction of neural signals and therefore associated with sensorimotor control. The question arises if symmetry or asymmetry in sensorimotor processing affects the development of symmetric or asymmetric motor programs in the lower extremities. The purpose of the study was to examine the mechanisms of the human mobility moto-control - the process of maintaining body balance in a standing position through an appropriate course of distribution of ground reaction forces in a time frame, in a situation requiring lower extremity movement symmetry. Methods. The autocorrelation function was calculated for ground reaction forces (in the three orthogonal axes) registered during 45 s of bipedal upright stance in two conditions (eyes open and closed). Results. Minor albeit significant deficiencies in postural muscle control were revealed as a function of time, as evidenced in the decay of the autocorrelation function to zero (T

Integrating multiple sensory systems to modulate neural networks controlling posture

In this study we investigated the ability of sensory input to produce tonic responses in hindlimb muscles to facilitate standing in adult spinal rats and tested two hypotheses: 1) whether the spinal neural networks below a complete spinal cord transection can produce tonic reactions by activating different sensory inputs and 2) whether facilitation of tonic and rhythmic responses via activation of afferents and with spinal cord stimulation could engage similar neuronal mechanisms. We used a dynamically controlled platform to generate vibration during weight bearing, epidural stimulation (at spinal cord level S1), and/or tail pinching to determine the postural control responses that can be generated by the lumbosacral spinal cord. We observed that a combination of platform displacement, epidural stimulation, and tail pinching produces a cumulative effect that progressively enhances tonic responses in the hindlimbs. Tonic responses produced by epidural stimulation alone during standing were represented mainly by monosynaptic responses, whereas the combination of epidural stimulation and tail pinching during standing or epidural stimulation during stepping on a treadmill facilitated bilaterally both monosynaptic and polysynaptic responses. The results demonstrate that tonic muscle activity after complete spinal cord injury can be facilitated by activation of specific combinations of afferent inputs associated with load-bearing proprioception and cutaneous input in the presence of epidural stimulation and indicate that whether activation of tonic or rhythmic responses is generated depends on the specific combinations of sources and types of afferents activated in the hindlimb muscles.

Task-specificity of balance training

Despite much research on balance training, it is still unclear whether balance training leads to highly task-specific adaptations or rather non-specific adaptations. Hence, in this study we examined whether balance training increased performance only in the balance task that was trained or also in non-trained tasks. Forty healthy participants (28 m 12 f, 25 ± 4 years, 177 ± 10 cm, 73 ± 14 kg) were assigned to one of two training groups (TGs) or a control group. Both TGs completed six sessions over 2 weeks, only the training device differed. Before and after the training, performance in the trained task as well as in additional untrained tasks was recorded. ANOVAs showed that each TG outperformed the other groups only in the task they had trained (e.g., task trained by TG1: +225% in TG1, only +41% and +30% in TG2 and control, group*time interaction, p<0.001; Untrained task 1: TG1 +48%, TG2 +48%, and control +30%, no significant interaction, p=0.72). In summary, 2 weeks of balance training resulted in highly task-specific effects, no transfer even to very similar tasks was observed. Therefore, we recommend identifying and training exactly those tasks that need improvement, and test the efficacy of training programs using specific tests instead of general tests with limited functional relevance.

Controlling legs for locomotion-insights from robotics and neurobiology

Walking is the most common terrestrial form of locomotion in animals. Its great versatility and flexibility has led to many attempts at building walking machines with similar capabilities. The control of walking is an active research area both in neurobiology and robotics, with a large and growing body of work. This paper gives an overview of the current knowledge on the control of legged locomotion in animals and machines and attempts to give walking control researchers from biology and robotics an overview of the current knowledge in both fields. We try to summarize the knowledge on the neurobiological basis of walking control in animals, emphasizing common principles seen in different species. In a section on walking robots, we review common approaches to walking controller design with a slight emphasis on biped walking control. We show where parallels between robotic and neurobiological walking controllers exist and how robotics and biology may benefit from each other. Finally, we discuss where research in the two fields diverges and suggest ways to bridge these gaps.

Effects of experimental insoles on body posture, mandibular kinematics and masticatory muscles activity. A pilot study in healthy volunteers

BACKGROUND

It has been hypothesized that different plantar sensory inputs could influence the whole body posture and dental occlusion but there is a lack of evidence on this possible association.

OBJECTIVES

To investigate the effects of experimental insoles redistributing plantar pressure on body posture, mandibular kinematics and electromyographic (EMG) activity of masticatory muscles on healthy subjects.

METHODS

A pilot study was conducted on 19 healthy volunteers that wore custom-made insoles normalizing the plantar pressure distribution for 2 weeks. Body posture parameters were measured by means of an optoelectronic stereophotogrammetric analysis; mandibular kinematics was analyzed by means of gothic arch tracings; superficial EMG activity of head and neck muscles was performed. Measurements were carried out 10 days before the insertion of the insoles, immediately before the insertion, the day after, 7 and 14 days after, in four different exteroceptive conditions.

RESULTS

The outcomes of the present study show that insoles do not modify significantly over time the parameters of body posture, SEMG activity of head and neck muscles and mandibular kinematics.

CONCLUSIONS

In this pilot study the experimental insoles did not significantly influence the body posture, the mandibular kinematics and the activity of masticatory muscles during a 14-day follow up period.

Putative spinal interneurons mediating postural limb reflexes provide a basis for postural control in different planes

The dorsal-side-up trunk orientation in standing quadrupeds is maintained by the postural system driven mainly by somatosensory inputs from the limbs. Postural limb reflexes (PLRs) represent a substantial component of this system. Earlier we described spinal neurons presumably contributing to the generation of PLRs. The first aim of the present study was to reveal trends in the distribution of neurons with different parameters of PLR-related activity across the gray matter of the spinal cord. The second aim was to estimate the contribution of PLR-related neurons with different patterns of convergence of sensory inputs from the limbs to stabilization of body orientation in different planes. For this purpose, the head and vertebral column of the decerebrate rabbit were fixed and the hindlimbs were positioned on a platform. Activity of individual neurons from L5 to L6 was recorded during PLRs evoked by lateral tilts of the platform. In addition, the neurons were tested by tilts of the platform under only the ipsilateral or only the contralateral limb, as well as during in-phase tilts of the platforms under both limbs. We found that, across the spinal gray matter, strength of PLR-related neuronal activity and sensory input from the ipsilateral limb decreased in the dorsoventral direction, while strength of the input from the contralateral limb increased. A near linear summation of tilt-related sensory inputs from different limbs was found. Functional roles were proposed for individual neurons. The obtained data present the first characterization of posture-related spinal neurons, forming a basis for studies of postural networks impaired by injury.

The single-leg-stance test in Parkinson's disease

Background

Timed single-leg-stance test (SLST) is widely used to assess postural control in the elderly. In Parkinson’s disease (PD), it has been shown that an SLST around 10 seconds or below may be a sensitive indicator of future falls. However, despite its role in fall risk, whether SLST times around 10 seconds marks a clinically important stage of disease progression has largely remained unexplored.

Methods

A cross-sectional study where 27 people with PD were recruited and instructed to undertake timed SLST for both legs was conducted. Disease motor impairment was assessed with the Unified Parkinson’s Disease Rating Scale Part 3 (UPDRS-III).

Results

This study found that: 1) the SLST in people with PD shows good test-retest reliability; 2) SLST values can be attributed to two non-overlapping clusters: a low (10.4 ± 6.3 seconds) and a high (47.6 ± 11.7 seconds) value SLST group; 3) only the low value SLST group can be considered abnormal when age-matched normative SLST data are taken into account for comparison; and 4) lower UPDRS-III motor performance, and the bradykinesia sub-score in particular, are only associated with the low SLST group.

Conclusion

These results lend further support that a low SLST time around 10 seconds marks a clinically important stage of disease progression with significant worsening of postural stability in PD.

Contribution of supraspinal systems to generation of automatic postural responses

Different species maintain a particular body orientation in space due to activity of the closed-loop postural control system. In this review we discuss the role of neurons of descending pathways in operation of this system as revealed in animal models of differing complexity: lower vertebrate (lamprey) and higher vertebrates (rabbit and cat). In the lamprey and quadruped mammals, the role of spinal and supraspinal mechanisms in the control of posture is different. In the lamprey, the system contains one closed-loop mechanism consisting of supraspino-spinal networks. Reticulospinal (RS) neurons play a key role in generation of postural corrections. Due to vestibular input, any deviation from the stabilized body orientation leads to activation of a specific population of RS neurons. Each of the neurons activates a specific motor synergy. Collectively, these neurons evoke the motor output necessary for the postural correction. In contrast to lampreys, postural corrections in quadrupeds are primarily based not on the vestibular input but on the somatosensory input from limb mechanoreceptors. The system contains two closed-loop mechanisms - spinal and spino-supraspinal networks, which supplement each other. Spinal networks receive somatosensory input from the limb signaling postural perturbations, and generate spinal postural limb reflexes. These reflexes are relatively weak, but in intact animals they are enhanced due to both tonic supraspinal drive and phasic supraspinal commands. Recent studies of these supraspinal influences are considered in this review. A hypothesis suggesting common principles of operation of the postural systems stabilizing body orientation in a particular plane in the lamprey and quadrupeds, that is interaction of antagonistic postural reflexes, is discussed.

Which way is up? Asymmetric spectral input along the dorsal-ventral axis influences postural responses in an amphibious annelid

Medicinal leeches are predatory annelids that exhibit countershading and reside in aquatic environments where light levels might be variable. They also leave the water and must contend with terrestrial environments. Yet, leeches generally maintain a dorsal upward position despite lacking statocysts. Leeches respond visually to both green and near-ultraviolet (UV) light. I used LEDs to test the hypothesis that ventral, but not dorsal UV would evoke compensatory movements to orient the body. Untethered leeches were tested using LEDs emitting at red (632 nm), green (513 nm), blue (455 nm) and UV (372 nm). UV light evoked responses in 100 % of trials and the leeches often rotated the ventral surface away from it. Visible light evoked no or modest responses (12-15 % of trials) and no body rotation. Electrophysiological recordings showed that ventral sensilla responded best to UV, dorsal sensilla to green. Additionally, a higher order interneuron that is engaged in a variety of parallel networks responded vigorously to UV presented ventrally, and both the visible and UV responses exhibited pronounced light adaptation. These results strongly support the suggestion that a dorsal light reflex in the leech uses spectral comparisons across the dorsal-ventral axis rather than, or in addition to, luminance.

Body stability and muscle and motor cortex activity during walking with wide stance

Biomechanical and neural mechanisms of balance control during walking are still poorly understood. In this study, we examined the body dynamic stability, activity of limb muscles, and activity of motor cortex neurons [primarily pyramidal tract neurons (PTNs)] in the cat during unconstrained walking and walking with a wide base of support (wide-stance walking). By recording three-dimensional full-body kinematics we found for the first time that during unconstrained walking the cat is dynamically unstable in the forward direction during stride phases when only two diagonal limbs support the body. In contrast to standing, an increased lateral between-paw distance during walking dramatically decreased the cat's body dynamic stability in double-support phases and prompted the cat to spend more time in three-legged support phases. Muscles contributing to abduction-adduction actions had higher activity during stance, while flexor muscles had higher activity during swing of wide-stance walking. The overwhelming majority of neurons in layer V of the motor cortex, 82% and 83% in the forelimb and hindlimb representation areas, respectively, were active differently during wide-stance walking compared with unconstrained condition, most often by having a different depth of stride-related frequency modulation along with a different mean discharge rate and/or preferred activity phase. Upon transition from unconstrained to wide-stance walking, proximal limb-related neuronal groups subtly but statistically significantly shifted their activity toward the swing phase, the stride phase where most of body instability occurs during this task. The data suggest that the motor cortex participates in maintenance of body dynamic stability during locomotion.

The WRAIR projectile concussive impact model of mild traumatic brain injury: re-design, testing and preclinical validation

The WRAIR projectile concussive impact (PCI) model was developed for preclinical study of concussion. It represents a truly non-invasive closed-head injury caused by a blunt impact. The original design, however, has several drawbacks that limit the manipulation of injury parameters. The present study describes engineering advancements made to the PCI injury model including helmet material testing, projectile impact energy/head kinematics and impact location. Material testing indicated that among the tested materials, 'fiber-glass/carbon' had the lowest elastic modulus and yield stress for providing an relative high percentage of load transfer from the projectile impact, resulting in significant hippocampal astrocyte activation. Impact energy testing of small projectiles, ranging in shape and size, showed the steel sphere produced the highest impact energy and the most consistent impact characteristics. Additional tests confirmed the steel sphere produced linear and rotational motions on the rat's head while remaining within a range that meets the criteria for mTBI. Finally, impact location testing results showed that PCI targeted at the temporoparietal surface of the rat head produced the most prominent gait abnormalities. Using the parameters defined above, pilot studies were conducted to provide initial validation of the PCI model demonstrating quantifiable and significant increases in righting reflex recovery time, axonal damage and astrocyte activation following single and multiple concussions.

Repeated Baclofen treatment ameliorates motor dysfunction, suppresses reflex activity and decreases the expression of signaling proteins in reticular nuclei and lumbar motoneurons after spinal trauma in rats

The interruption of supraspinal input to the spinal cord leads to motor dysfunction and the development of spasticity. Clinical studies have shown that Baclofen (a GABAB agonist), while effective in modulating spasticity is associated with side-effects and the development of tolerance. The aim of the present study was to assess if discontinued Baclofen treatment and its repeated application leads antispasticity effects, and whether such changes affect neuronal nitric oxide synthase (nNOS) in the brainstem, nNOS and parvalbumin (PV) in lumbar α-motoneurons and glial fibrillary acidic protein in the ventral horn of the spinal cord. Adult male Wistar rats were exposed to Th9 spinal cord transection. Baclofen (30mg/b.w.) diluted in drinking water, was administered for 6 days, starting at week 1 after injury and then repeated till week 4 after injury. The behavior of the animals was tested (tail-flick test, BBB locomotor score) from 1 to 8 weeks. Our results clearly indicate the role of nitric oxide, produced by nNOS in the initiation and the maintenance of spasticity states 1, 6 and 8 weeks after spinal trauma. A considerable decrease of nNOS staining after Baclofen treatment correlates with improvement of motor dysfunction. The findings also show that parvalbumin and astrocytes participate in the regulation of ion concentrations in the sub-acute phase after the injury.

Effects of galvanic vestibular stimulation on postural limb reflexes and neurons of spinal postural network

Quadrupeds maintain the dorsal side up body orientation due to the activity of the postural control system driven by limb mechanoreceptors. Binaural galvanic vestibular stimulation (GVS) causes a lateral body sway toward the anode. Previously, we have shown that this new position is actively stabilized, suggesting that GVS changes a set point in the reflex mechanisms controlling body posture. The aim of the present study was to reveal the underlying neuronal mechanisms. Experiments were performed on decerebrate rabbits. The vertebral column was rigidly fixed, whereas hindlimbs were positioned on a platform. Periodic lateral tilts of the platform caused postural limb reflexes (PLRs): activation of extensors in the loaded and flexing limb and a decrease in extensor activity in the opposite (unloaded and extending) limb. Putative spinal interneurons were recorded in segments L4-L5 during PLRs, with and without GVS. We have found that GVS enhanced PLRs on the cathode side and reduced them on the anode side. This asymmetry in PLRs can account for changes in the stabilized body orientation observed in normal rabbits subjected to continuous GVS. Responses to platform tilts (frequency modulation) were observed in 106 spinal neurons, suggesting that they can contribute to PLR generation. Two neuron groups were active in opposite phases of the tilt cycle of the ipsi-limb: F-neurons in the flexion phase, and E-neurons in the extension phase. Neurons were driven mainly by afferent input from the ipsi-limb. If one supposes that F- and E-neurons contribute, respectively, to excitation and inhibition of extensor motoneurons, one can expect that the pattern of response to GVS in F-neurons will be similar to that in extensor muscles, whereas E-neurons will have an opposite pattern. We have found that ~40% of all modulated neurons meet this condition, suggesting that they contribute to the generation of PLRs and to the GVS-caused changes in PLRs.

Somatosensory control of balance during locomotion in decerebrated cat

Postmammillary decerebrated cats can generate stepping on a moving treadmill belt when the brain stem or spinal cord is stimulated tonically and the hindquarters are supported both vertically and laterally. While adequate propulsion seems to be generated by the hindlimbs under these conditions, the ability to sustain equilibrium during locomotion has not been examined extensively. We found that tonic epidural spinal cord stimulation (5 Hz at L5) of decerebrated cats initiated and sustained unrestrained weight-bearing hindlimb stepping for extended periods. Detailed analyses of the relationships among hindlimb muscle EMG activity and trunk and limb kinematics and kinetics indicated that the motor circuitries in decerebrated cats actively maintain equilibrium during walking, similar to that observed in intact animals. Because of the suppression of vestibular, visual, and head-neck-trunk sensory input, balance-related adjustments relied entirely on the integration of somatosensory information arising from the moving hindquarters. In addition to dynamic balance control during unperturbed locomotion, sustained stepping could be reestablished rapidly after a collapse or stumble when the hindquarters switched from a restrained to an unrestrained condition. Deflecting the body by pulling the tail laterally induced adaptive modulations in the EMG activity, step cycle features, and left-right ground reaction forces that were sufficient to maintain lateral stability. Thus the brain stem-spinal cord circuitry of decerebrated cats in response to tonic spinal cord stimulation can control dynamic balance during locomotion using only somatosensory input.

Physiological and circuit mechanisms of postural control

The postural system maintains a specific body orientation and equilibrium during standing and during locomotion in the presence of many destabilizing factors (external and internal). Numerous studies in humans have revealed essential features of the functional organization of this system. Recent studies on different animal models have significantly supplemented human studies. They have greatly expanded our knowledge of how the control system operates, how the postural functions are distributed within different parts of CNS, and how these parts interact with each other to produce postural corrections and adjustments. This review outlines recent advances in the studies of postural control in quadrupeds, with special attention given the neuronal postural mechanisms.