Thomas Graham Brown (1882--1965), Anders Lundberg (1920-), and the neural control of stepping

Operation regimes of spinal circuits controlling locomotion and the role of supraspinal drives and sensory feedback

Locomotion in mammals is directly controlled by the spinal neuronal network, operating under the control of supraspinal signals and somatosensory feedback that interact with each other. However, the functional architecture of the spinal locomotor network, its operation regimes, and the role of supraspinal and sensory feedback in different locomotor behaviors, including at different speeds, remain unclear. We developed a computational model of spinal locomotor circuits receiving supraspinal drives and limb sensory feedback that could reproduce multiple experimental data obtained in intact and spinal-transected cats during tied-belt and split-belt treadmill locomotion. We provide evidence that the spinal locomotor network operates in different regimes depending on locomotor speed. In an intact system, at slow speeds (<0.4 m/s), the spinal network operates in a non-oscillating state-machine regime and requires sensory feedback or external inputs for phase transitions. Removing sensory feedback related to limb extension prevents locomotor oscillations at slow speeds. With increasing speed and supraspinal drives, the spinal network switches to a flexor-driven oscillatory regime and then to a classical half-center regime. Following spinal transection, the model predicts that the spinal network can only operate in the state-machine regime. Our results suggest that the spinal network operates in different regimes for slow exploratory and fast escape locomotor behaviors, making use of different control mechanisms.

Inhibitory circuits generate rhythms for leg movements during Drosophila grooming

Limbs execute diverse actions coordinated by the nervous system through multiple motor programs. The basic architecture of motor neurons that activate muscles that articulate joints for antagonistic flexion and extension movements is conserved from flies to vertebrates. While excitatory premotor circuits are expected to establish sets of leg motor neurons that work together, our study uncovered a new instructive role for inhibitory circuits: their ability to generate rhythmic leg movements. Using electron microscopy data for the Drosophila nerve cord, we categorized ~120 GABAergic inhibitory neurons from the 13A and 13B hemi-lineages into classes based on similarities in morphology and connectivity. By mapping their synaptic partners, we uncovered pathways for inhibiting specific groups of motor neurons, disinhibiting antagonistic counterparts, and inducing alternation between flexion and extension. We tested the function of specific inhibitory neurons through optogenetic activation and silencing, using an in-depth ethological analysis of leg movements during grooming. We combined anatomy and behavior analysis findings to construct a computational model that can reproduce major aspects of the observed behavior, confirming the sufficiency of these premotor inhibitory circuits to generate rhythms.

Sensory feedback and central neuronal interactions in mouse locomotion

Locomotion is a complex process involving specific interactions between the central neural controller and the mechanical components of the system. The basic rhythmic activity generated by locomotor circuits in the spinal cord defines rhythmic limb movements and their central coordination. The operation of these circuits is modulated by sensory feedback from the limbs providing information about the state of the limbs and the body. However, the specific role and contribution of central interactions and sensory feedback in the control of locomotor gait and posture remain poorly understood. We use biomechanical data on quadrupedal locomotion in mice and recent findings on the organization of neural interactions within the spinal locomotor circuitry to create and analyse a tractable mathematical model of mouse locomotion. The model includes a simplified mechanical model of the mouse body with four limbs and a central controller composed of four rhythm generators, each operating as a state machine controlling the state of one limb. Feedback signals characterize the load and extension of each limb as well as postural stability (balance). We systematically investigate and compare several model versions and compare their behaviour to existing experimental data on mouse locomotion. Our results highlight the specific roles of sensory feedback and some central propriospinal interactions between circuits controlling fore and hind limbs for speed-dependent gait expression. Our models suggest that postural imbalance feedback may be critically involved in the control of swing-to-stance transitions in each limb and the stabilization of walking direction.

Operation regimes of spinal circuits controlling locomotion and role of supraspinal drives and sensory feedback

Locomotion in mammals is directly controlled by the spinal neuronal network, operating under the control of supraspinal signals and somatosensory feedback that interact with each other. However, the functional architecture of the spinal locomotor network, its operation regimes, and the role of supraspinal and sensory feedback in different locomotor behaviors, including at different speeds, remain unclear. We developed a computational model of spinal locomotor circuits receiving supraspinal drives and limb sensory feedback that could reproduce multiple experimental data obtained in intact and spinal-transected cats during tied-belt and split-belt treadmill locomotion. We provide evidence that the spinal locomotor network operates in different regimes depending on locomotor speed. In an intact system, at slow speeds (< 0.4 m/s), the spinal network operates in a non-oscillating state-machine regime and requires sensory feedback or external inputs for phase transitions. Removing sensory feedback related to limb extension prevents locomotor oscillations at slow speeds. With increasing speed and supraspinal drives, the spinal network switches to a flexor-driven oscillatory regime and then to a classical half-center regime. Following spinal transection, the model predicts that the spinal network can only operate in the state-machine regime. Our results suggest that the spinal network operates in different regimes for slow exploratory and fast escape locomotor behaviors, making use of different control mechanisms.

Mobile neuroimaging: What we have learned about the neural control of human walking, with an emphasis on EEG-based research

Our understanding of the neural control of human walking has changed significantly over the last twenty years and mobile brain imaging methods have contributed substantially to current knowledge. High-density electroencephalography (EEG) has the advantages of being lightweight and mobile while providing temporal resolution of brain changes within a gait cycle. Advances in EEG hardware and processing methods have led to a proliferation of research on the neural control of locomotion in neurologically intact adults. We provide a narrative review of the advantages and disadvantages of different mobile brain imaging methods, then summarize findings from mobile EEG studies quantifying electrocortical activity during human walking. Contrary to historical views on the neural control of locomotion, recent studies highlight the widespread involvement of many areas, such as the anterior cingulate, posterior parietal, prefrontal, premotor, sensorimotor, supplementary motor, and occipital cortices, that show active fluctuations in electrical power during walking. The electrocortical activity changes with speed, stability, perturbations, and gait adaptation. We end with a discussion on the next steps in mobile EEG research.

Tapping into the human spinal locomotor centres with transspinal stimulation

Human locomotion is controlled by spinal neuronal networks of similar properties, function, and organization to those described in animals. Transspinal stimulation affects the spinal locomotor networks and is used to improve standing and walking ability in paralyzed people. However, the function of locomotor centers during transspinal stimulation at different frequencies and intensities is not known. Here, we document the 3D joint kinematics and spatiotemporal gait characteristics during transspinal stimulation at 15, 30, and 50 Hz at sub-threshold and supra-threshold stimulation intensities. We document the temporal structure of gait patterns, dynamic stability of joint movements over stride-to-stride fluctuations, and limb coordination during walking at a self-selected speed in healthy subjects. We found that transspinal stimulation (1) affects the kinematics of the hip, knee, and ankle joints, (2) promotes a more stable coordination at the left ankle, (3) affects interlimb coordination of the thighs, and (4) intralimb coordination between thigh and foot, (5) promotes greater dynamic stability of the hips, (6) increases the persistence of fluctuations in step length variability, and lastly (7) affects mechanical walking stability. These results support that transspinal stimulation is an important neuromodulatory strategy that directly affects gait symmetry and dynamic stability. The conservation of main effects at different frequencies and intensities calls for systematic investigation of stimulation protocols for clinical applications.

Updating perspectives on spinal cord function: motor coordination, timing, relational processing, and memory below the brain

Those studying neural systems within the brain have historically assumed that lower-level processes in the spinal cord act in a mechanical manner, to relay afferent signals and execute motor commands. From this view, abstracting temporal and environmental relations is the province of the brain. Here we review work conducted over the last 50 years that challenges this perspective, demonstrating that mechanisms within the spinal cord can organize coordinated behavior (stepping), induce a lasting change in how pain (nociceptive) signals are processed, abstract stimulus-stimulus (Pavlovian) and response-outcome (instrumental) relations, and infer whether stimuli occur in a random or regular manner. The mechanisms that underlie these processes depend upon signal pathways (e.g., NMDA receptor mediated plasticity) analogous to those implicated in brain-dependent learning and memory. New data show that spinal cord injury (SCI) can enable plasticity within the spinal cord by reducing the inhibitory effect of GABA. It is suggested that the signals relayed to the brain may contain information about environmental relations and that spinal cord systems can coordinate action in response to descending signals from the brain. We further suggest that the study of stimulus processing, learning, memory, and cognitive-like processing in the spinal cord can inform our views of brain function, providing an attractive model system. Most importantly, the work has revealed new avenues of treatment for those that have suffered a SCI.

Sensory Feedback and Central Neuronal Interactions in Mouse Locomotion

Locomotion is a complex process involving specific interactions between the central neural controller and the mechanical components of the system. The basic rhythmic activity generated by locomotor circuits in the spinal cord defines rhythmic limb movements and their central coordination. The operation of these circuits is modulated by sensory feedback from the limbs providing information about the state of the limbs and the body. However, the specific role and contribution of central interactions and sensory feedback in the control of locomotor gait and posture remain poorly understood. We use biomechanical data on quadrupedal locomotion in mice and recent findings on the organization of neural interactions within the spinal locomotor circuitry to create and analyze a tractable mathematical model of mouse locomotion. The model includes a simplified mechanical model of the mouse body with four limbs and a central controller composed of four rhythm generators, each operating as a state machine controlling the state of one limb. Feedback signals characterize the load and extension of each limb as well as postural stability (balance). We systematically investigate and compare several model versions and compare their behavior to existing experimental data on mouse locomotion. Our results highlight the specific roles of sensory feedback and some central propriospinal interactions between circuits controlling fore and hind limbs for speed-dependent gait expression. Our models suggest that postural imbalance feedback may be critically involved in the control of swing-to-stance transitions in each limb and the stabilization of walking direction.

Locomotor pattern generation and descending control: a historical perspective

The ability to generate and control locomotor movements depends on complex interactions between many areas of the nervous system, the musculoskeletal system, and the environment. How the nervous system manages to accomplish this task has been the subject of investigation for more than a century. In vertebrates, locomotion is generated by neural networks located in the spinal cord referred to as central pattern generators. Descending inputs from the brain stem initiate, maintain, and stop locomotion as well as control speed and direction. Sensory inputs adapt locomotor programs to the environmental conditions. This review presents a comparative and historical overview of some of the neural mechanisms underlying the control of locomotion in vertebrates. We have put an emphasis on spinal mechanisms and descending control.

Spinal cords: Symphonies of interneurons across species

Vertebrate movement is orchestrated by spinal inter- and motor neurons that, together with sensory and cognitive input, produce dynamic motor behaviors. These behaviors vary from the simple undulatory swimming of fish and larval aquatic species to the highly coordinated running, reaching and grasping of mice, humans and other mammals. This variation raises the fundamental question of how spinal circuits have changed in register with motor behavior. In simple, undulatory fish, exemplified by the lamprey, two broad classes of interneurons shape motor neuron output: ipsilateral-projecting excitatory neurons, and commissural-projecting inhibitory neurons. An additional class of ipsilateral inhibitory neurons is required to generate escape swim behavior in larval zebrafish and tadpoles. In limbed vertebrates, a more complex spinal neuron composition is observed. In this review, we provide evidence that movement elaboration correlates with an increase and specialization of these three basic interneuron types into molecularly, anatomically, and functionally distinct subpopulations. We summarize recent work linking neuron types to movement-pattern generation across fish, amphibians, reptiles, birds and mammals.

Comparison of the Effects of Isometric Horizontal Abduction on Shoulder Muscle Activity During Wall Push-Up Plus and Wall Slide in Individuals With Scapular Winging

CONTEXT

Wall push-up plus (WPP) and wall slide (WS) are commonly prescribed in early rehabilitation to increase serratus anterior (SA) muscle activity. For individuals with scapular winging (SW), synergistic muscles such as upper trapezius (UT) and pectoralis major (PM) may compensate for weak SA during scapular movement. However, no studies have applied isometric horizontal abduction (IHA) during WS in individuals with SW nor have compared it with WPP with IHA.

OBJECTIVES

This study compared the effects of IHA on shoulder muscle activity during WPP and WS exercises in individuals with SW.

DESIGN

Cross-sectional study; 2-way repeated-measures analysis of variance was used to assess the statistical significance of observed differences in SA, UT, PM, lower trapezius (LT), and infraspinatus (IS) muscle activities.

SETTING

Research laboratory.

PATIENTS

We recruited 30 individuals with SW comprising 20 men and 10 women.

INTERVENTIONS

The individuals performed WPP and WS exercises with and without IHA using Thera-Band.

MAIN OUTCOME MEASURES

Surface electromyography was used to measure muscle activity of the SA, UT, PM, LT, and IS during the isometric phase of WPP and WS. Maximal voluntary isometric contractions were recorded to normalize electromyographic data.

RESULTS

There was no significant interaction between IHA application and exercise type for any of the shoulder muscles. IHA application increased SA (P = .008), UT (P = .001), LT (P = .009), and IS (P = .000) activities and decreased PM (P = .001) activity compared with those without IHA. WS exercises elicited higher PM (P = .017) and LT (P = .011) activities than WPP.

CONCLUSION

WPP and WS with IHA may be effective in increasing the muscle activities of shoulder stabilizers and preventing overactivation of PM activity. WPP may be recommended for individuals with overactivated PM, whereas WS may be used to increase LT activity.

Neurobiological reduction: From cellular explanations of behavior to interventions

Scientific reductionism, the view that higher level functions can be explained by properties at some lower-level or levels, has been an assumption of nervous system analyses since the acceptance of the neuron doctrine in the late 19th century, and became a dominant experimental approach with the development of intracellular recording techniques in the mid-20th century. Subsequent refinements of electrophysiological approaches and the continual development of molecular and genetic techniques have promoted a focus on molecular and cellular mechanisms in experimental analyses and explanations of sensory, motor, and cognitive functions. Reductionist assumptions have also influenced our views of the etiology and treatment of psychopathologies, and have more recently led to claims that we can, or even should, pharmacologically enhance the normal brain. Reductionism remains an area of active debate in the philosophy of science. In neuroscience and psychology, the debate typically focuses on the mind-brain question and the mechanisms of cognition, and how or if they can be explained in neurobiological terms. However, these debates are affected by the complexity of the phenomena being considered and the difficulty of obtaining the necessary neurobiological detail. We can instead ask whether features identified in neurobiological analyses of simpler aspects in simpler nervous systems support current molecular and cellular approaches to explaining systems or behaviors. While my view is that they do not, this does not invite the opposing view prevalent in dichotomous thinking that molecular and cellular detail is irrelevant and we should focus on computations or representations. We instead need to consider how to address the long-standing dilemma of how a nervous system that ostensibly functions through discrete cell to cell communication can generate population effects across multiple spatial and temporal scales to generate behavior.

Spinal automaticity of movement control and its role in recovering function after spinal injury

INTRODUCTION

The significance of the spinal circuitry in controlling postural and locomotor functions largely re-emerged in the mid-1970s under the leadership of Sten Grillner, demonstrating key phenomena of "central pattern generation" and "fictive locomotion" with an evolutionary perspective. These concepts raised the question of how much function can be recovered after paralysis, given the intrinsic automaticity of spinal networks in injured and uninjured states in adults.

AREAS COVERED

This review explores biological mechanisms governing spinal control of movements such as posture and locomotion. We focus on concepts that have evolved from experiments performed over the past decade. Rather than a comprehensive review of the vast literature on the neural control of posture and locomotion, we focus on the various mechanisms underlying functional automaticity, and their clinical relevance.

EXPERT OPINION

We propose that multiple combinations of sensory mechanoreceptors linked to proprioception generate an infinite number of different sensory ensembles, having species-specific meaning and extensive influence in controlling posture and locomotion. These sensory ensembles are translated as a probabilistic phenomenon into highly specific but indeterminate actions. Therefore, we opine that spinal translation of these ensembles in real-time plays a central role in the automaticity of motor control in individuals with and without severe neuromotor dysfunction.

Soleus H and Lower Limb Posterior Root Muscle Reflexes During Stepping After Incomplete SCI

The goal of this study was to examine and compare the step cycle related modulation of the soleus H and posterior root muscle (PRM) reflexes in subjects with and without spinal cord injury. Ten subjects without neurological injury and fifteen subjects with spinal cord injury (SCI) underwent soleus H reflex and lower limb PRM reflex testing while standing and stepping in a robotic gait orthosis. Reflex amplitudes were evaluated during standing, mid stance and mid swing to determine if speed and/or injury altered step cycle related neuromodulation. H and PRM reflexes in the soleus underwent step cycle related modulation in injured and uninjured subjects though the degree of modulation differed between the two reflexes with the H reflex showing more step cycle related modulation. We found in the SCI group that both the soleus H and soleus PRM reflex amplitudes were higher relative to the non-injured group and modulated less during the step cycle. We also found that modulation of the soleus H reflex, but not soleus PRM reflex, correlated to the lower extremity motor scores in individuals with SCI. Our evidence suggests that the inability to provide appropriate step cycle related reflex modulation may be due to decreased supra-spinal regulation of motoneuron and spinal excitability and could be an indicator of the severity of injury as it relates to clinically measured lower extremity motor scores.

The Mesencephalic Locomotor Region: Beyond Locomotor Control

The mesencephalic locomotor region (MLR) was discovered several decades ago in the cat. It was functionally defined based on the ability of low threshold electrical stimuli within a region comprising the cuneiform and pedunculopontine nucleus to evoke locomotion. Since then, similar regions have been found in diverse vertebrate species, including the lamprey, skate, rodent, pig, monkey, and human. The MLR, while often viewed under the lens of locomotion, is involved in diverse processes involving the autonomic nervous system, respiratory system, and the state-dependent activation of motor systems. This review will discuss the pedunculopontine nucleus and cuneiform nucleus that comprises the MLR and examine their respective connectomes from both an anatomical and functional angle. From a functional perspective, the MLR primes the cardiovascular and respiratory systems before the locomotor activity occurs. Inputs from a variety of higher structures, and direct outputs to the monoaminergic nuclei, allow the MLR to be able to respond appropriately to state-dependent locomotion. These state-dependent effects are roughly divided into escape and exploratory behavior, and the MLR also can reinforce the selection of these locomotor behaviors through projections to adjacent structures such as the periaqueductal gray or to limbic and cortical regions. Findings from the rat, mouse, pig, and cat will be discussed to highlight similarities and differences among diverse species.

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.

Unexpected Terrain Induced Changes in Cortical Activity in Bipedal-Walking Rats

Simple Summary

Most studies on cortical dynamics during walking require subjects to walk stably on specific terrain. In fact, humans or other animals are often disturbed by an abrupt change in terrains during walking. To study the impact of unexpected terrain on cortical activity, we analyzed the kinematics and electroencephalography (EEG) dynamics of bipedal-walking rats after encountering unexpected terrain. We found that the gait of rats after encountering the unexpected terrain were significantly different from normal walking. Furthermore, the activities of the left and right primary motor areas (M1), the left and right primary somatosensory areas (S1), and the retrosplenial area (RSP) are coupled to gait cycle phase and varied with the terrain conditions. These findings suggest that unexpected terrains induced changes in gait and cortical activity, and provide novel insights into cortical dynamics during walking.

Abstract

Humans and other animals can quickly respond to unexpected terrains during walking, but little is known about the cortical dynamics in this process. To study the impact of unexpected terrains on brain activity, we allowed rats with blocked vision to walk on a treadmill in a bipedal posture and then walk on an uneven area at a random position on the treadmill belt. Whole brain EEG signals and hind limb kinematics of bipedal-walking rats were recorded. After encountering unexpected terrain, the θ band power of the bilateral M1, the γ band power of the left S1, and the θ to γ band power of the RSP significantly decreased compared with normal walking. Furthermore, when the rats left uneven terrain, the β band power of the bilateral M1 and the α band power of the right M1 decreased, while the γ band power of the left M1 significantly increased compared with normal walking. Compared with the flat terrain, the θ to low β (3–20 Hz) band power of the bilateral S1 increased after the rats contacted the uneven terrain and then decreased in the single- or double- support phase. These results support the hypothesis that unexpected terrains induced changes in cortical activity.

Epidural spinal cord stimulation as an intervention for motor recovery after motor complete spinal cord injury

Spinal cord injury (SCI) commonly results in permanent loss of motor, sensory, and autonomic function. Recent clinical studies have shown that epidural spinal cord stimulation may provide a beneficial adjunct for restoring lower extremity and other neurological functions. Herein, we review the recent clinical advances of lumbosacral epidural stimulation for restoration of sensorimotor function in individuals with motor complete SCI and we discuss the putative neural pathways involved in this promising neurorehabilitative approach. We focus on three main sections: review recent clinical results for locomotor restoration in complete SCI; discuss the contemporary understanding of electrical neuromodulation and signal transduction pathways involved in spinal locomotor networks; and review current challenges of motor system modulation and future directions toward integrative neurorestoration. The current understanding is that initial depolarization occurs at the level of large diameter dorsal root proprioceptive afferents that when integrated with interneuronal and latent residual supraspinal translesional connections can recruit locomotor centers and augment downstream motor units. Spinal epidural stimulation can initiate excitability changes in spinal networks and supraspinal networks. Different stimulation parameters can facilitate standing or stepping, and it may also have potential for augmenting myriad other sensorimotor and autonomic functions. More comprehensive investigation of the mechanisms that mediate the transformation of dysfunctional spinal networks to higher functional states with a greater focus on integrated systems-based control system may reveal the key mechanisms underlying neurological augmentation and motor restoration after severe paralysis.

An intracortical neuroprosthesis immediately alleviates walking deficits and improves recovery of leg control after spinal cord injury

Most rehabilitation interventions after spinal cord injury (SCI) only target the sublesional spinal networks, peripheral nerves, and muscles. However, mammalian locomotion is not a mere act of rhythmic pattern generation. Recovery of cortical control is essential for voluntary movement and modulation of gait. We developed an intracortical neuroprosthetic intervention to SCI, with the goal to condition cortical locomotor control. Neurostimulation delivered in phase coherence with ongoing locomotion immediately alleviated primary SCI deficits, such as leg dragging, in rats with incomplete SCI. Cortical neurostimulation achieved high fidelity and markedly proportional online control of leg trajectories in both healthy and SCI rats. Long-term neuroprosthetic training lastingly improved cortical control of locomotion, whereas short training held transient improvements. We performed longitudinal awake cortical motor mapping, unveiling that recovery of cortico-spinal transmission tightly parallels return of locomotor function in rats. These results advocate directly targeting the motor cortex in clinical neuroprosthetic approaches.

Computational Modeling of Spinal Locomotor Circuitry in the Age of Molecular Genetics

Neuronal circuits in the spinal cord are essential for the control of locomotion. They integrate supraspinal commands and afferent feedback signals to produce coordinated rhythmic muscle activations necessary for stable locomotion. For several decades, computational modeling has complemented experimental studies by providing a mechanistic rationale for experimental observations and by deriving experimentally testable predictions. This symbiotic relationship between experimental and computational approaches has resulted in numerous fundamental insights. With recent advances in molecular and genetic methods, it has become possible to manipulate specific constituent elements of the spinal circuitry and relate them to locomotor behavior. This has led to computational modeling studies investigating mechanisms at the level of genetically defined neuronal populations and their interactions. We review literature on the spinal locomotor circuitry from a computational perspective. By reviewing examples leading up to and in the age of molecular genetics, we demonstrate the importance of computational modeling and its interactions with experiments. Moving forward, neuromechanical models with neuronal circuitry modeled at the level of genetically defined neuronal populations will be required to further unravel the mechanisms by which neuronal interactions lead to locomotor behavior.

Brainless Walking: Animal Gaits Emerge From an Actuator Characteristic

In this study, we discovered a phenomenon in which a quadruped robot without any sensors or microprocessor can autonomously generate the various gait patterns of animals using actuator characteristics and select the gaits according to the speed. The robot has one DC motor on each limb and a slider-crank mechanism connected to the motor shaft. Since each motor is directly connected to a power supply, the robot only moves its foot on an elliptical trajectory under a constant voltage. Although this robot does not have any computational equipment such as sensors or microprocessors, when we applied a voltage to the motor, each limb begins to adjust its gait autonomously and finally converged to a steady gait pattern. Furthermore, by raising the input voltage from the power supply, the gait changed from a pace to a half-bound, according to the speed, and also we observed various gait patterns, such as a bound or a rotary gallop. We investigated the convergence property of the gaits for several initial states and input voltages and have described detailed experimental results of each gait observed.

EEG differentiates left and right imagined Lower Limb movement

BACKGROUND

Identifying which EEG signals distinguish left from right leg movements in imagined lower limb movement is crucial to building an effective and efficient brain-computer interface (BCI). Past findings on this issue have been mixed, partly due to the difficulty in collecting and isolating the relevant information. The purpose of this study was to contribute to this new and important literature.

RESEARCH QUESTION

Can left versus right imagined stepping be differentiated using the alpha, beta, and gamma frequencies of EEG data at four electrodes (C1, C2, PO3, and PO4)?

METHODS

An experiment was conducted with a sample of 16 healthy male participants. They imagined left and right lower limb movements across 60 trials at two time periods separated by one week. Participants were fitted with a 64-electrode headcap, lay supine on a specially designed device and then completed the imagined task while observing a customized computer-generated image of a human walking to signify the left and right steps, respectively.

RESULTS

Findings showed that eight of the twelve frequency bands from 4 EEG electrodes were significant in differentiating imagined left from right lower limb movement. Using these data points, a neural network analysis resulted in an overall participant average test classification accuracy of left versus right movements at 63 %.

SIGNIFICANCE

Our study provides support for using the alpha, beta and gamma frequency bands at the sensorimotor areas (C1 and C2 electrodes) and incorporating information from the parietal/occipital lobes (PO3 and PO4 electrodes) for focused, real-time EEG signal processing to assist in creating a BCI for those with lower limb compromised mobility.

Electrocortical activity in freely walking rats varies with environmental conditions

Longstanding theories in the field of neurophysiology have held that walking in rats is an unconscious, rhythmic locomotion that does not require cortical involvement. However, recent studies have suggested that the extent of cortical involvement during walking actually varies depending on the environmental conditions. To determine the impact of environmental conditions on cortical engagement in freely walking rats, we recorded limb kinematics and signals from implanted electroencephalography arrays in rats performing a series of natural behaviors. We found that rat gaits were significantly different across various locomotion terrains (e.g. walking on an upslope vs. downslope). Further, rat forelimbs and hindlimbs showed similar patterns of motion. The results also suggested that rat cortical engagement during walking varied across environmental conditions. Specifically, α band power significantly increased during 30° downslope walking in the posterior parietal, left secondary motor, and left somatosensory clusters. Additionally, during 30° upslope walking, the β band power was greater in the left primary motor and left and right secondary motor sources. Further, rats walking on up- or downslopes of varying steepness were found to have different cortical activities. Compared with 10° downslope walking, α band power was greater during 30° downslope locomotion in the left primary motor and somatosensory sources. These findings support the hypothesis that cortical contribution during walking in rats is influenced by environmental conditions, underlining the importance of goal-directed behaviors for motor function rehabilitation and neuro-prosthetic control in brain-machine interfaces.

On the Organization of the Locomotor CPG: Insights From Split-Belt Locomotion and Mathematical Modeling

Rhythmic limb movements during locomotion are controlled by central pattern generator (CPG) circuits located in the spinal cord. It is considered that these circuits are composed of individual rhythm generators (RGs) for each limb interacting with each other through multiple commissural and long propriospinal circuits. The organization and operation of each RG are not fully understood, and different competing theories exist about interactions between its flexor and extensor components, as well as about left-right commissural interactions between the RGs. The central idea of circuit organization proposed in this study is that with an increase of excitatory input to each RG (or an increase in locomotor speed) the rhythmogenic mechanism of the RGs changes from "flexor-driven" rhythmicity to a "classical half-center" mechanism. We test this hypothesis using our experimental data on changes in duration of stance and swing phases in the intact and spinal cats walking on the ground or tied-belt treadmills (symmetric conditions) or split-belt treadmills with different left and right belt speeds (asymmetric conditions). We compare these experimental data with the results of mathematical modeling, in which simulated CPG circuits operate in similar symmetric and asymmetric conditions with matching or differing control drives to the left and right RGs. The obtained results support the proposed concept of state-dependent changes in RG operation and specific commissural interactions between the RGs. The performed simulations and mathematical analysis of model operation under different conditions provide new insights into CPG network organization and limb coordination during locomotion.

Differentiating the Brain's involvement in Executed and Imagined Stepping using fMRI

The purpose of this study was to extend the extant literature regarding brain areas that are activated during executed and imagined lower limb movement. Past research suggests that stepping, as a cyclical movement, should activate the motor control areas of the brain that integrates smooth movements with spinal cord nerves. The neuronal activity needed to imagine that same activity is likely to recruit additional sensory-motor areas that provide initiation and inhibition signals, making this task take on a neuronal activity pattern that is more similar to discrete movements. To assess this research question, 16 participants took part in the current study where they executed and imagined stepping, with movement at the hip, knee, and ankle joints, while viewing a computer-generated image of a human walking. A block design with a total of 10 blocks for rest and task for each condition was used. Rest blocks lasted 18 seconds, followed by an 18-second display of the visual stimulus. Results showed that in the executed condition, areas of the brain that are most prominently associated with sensory-motor activity were activated. In the imagined condition areas of the brain associated with movement control, inhibition of movement, and the integration of sensory input and motor output (parietal and occipital) were also activated. These findings contribute to the literature identifying brain areas that are activated in lower limb locomotion.

Sensory Feedback Control of Locomotor Pattern Generation in Cats and Mice

Traditionally, research aimed at the understanding of the sensory control of terrestrial mammalian locomotion has focused on cats as the animal model. But advances in molecular genetics and new methods to record movement in small animals have moved mice into the forefront of locomotor research. In this review article, I will first give an overview of what is known about sensory feedback control of locomotion, mainly emerged from experiments performed on cats. This overview will not be an exhaustive overview, but will rather aim to give a broad picture of what has been learned about the sensory control of locomotion using cats as the animal model. I will then give a brief summary of how the mouse is adding to these insights.

Development of a sexual dimorphism in a central pattern generator driving a rhythmic behavior: The role of glia-mediated potassium buffering in the pacemaker nucleus of the weakly electric fish Apteronotus leptorhynchus

Central pattern generators play a critical role in the neural control of rhythmic behaviors. One of their characteristic features is the ability to modulate the oscillatory output. An important yet little-studied type of modulation involves the generation of oscillations that are sexually dimorphic in frequency. In the weakly electric fish Apteronotus leptorhynchus, the pacemaker nucleus serves as a central pattern generator that drives the electric organ discharge of the fish in a one-to-one fashion. Males discharge at higher frequencies than females-a sexual dimorphism that develops under the influence of steroid hormones. The two principal neurons that constitute the oscillatory network of the pacemaker nucleus are the pacemaker and relay cells. Whereas the number and size of the pacemaker and relay cells are sexually monomorphic, pronounced sex-dependent differences exist in the morphology, and subcellular properties of astrocytes, which form a syncytium closely associated with these neurons. In females, compared to males, the astrocytic syncytium covers a larger area surrounding the pacemaker and relay cells and exhibits higher levels of expression of connexin-43 expression. The latter indicates a strong gap-junction coupling of the individual cells within the syncytium. It is hypothesized that these sex-specific differences result in an increased capacity for buffering of extracellular potassium ions, thereby lowering the potassium equilibrium potential, which, in turn, leads to a decrease in the oscillation frequency. This hypothesis has received strong support from simulations based on computational models of individual neurons and the whole neural network of the pacemaker nucleus.

Faster Gait Speeds Reduce Alpha and Beta EEG Spectral Power From Human Sensorimotor Cortex

OBJECTIVE

Our aim was to determine if walking speed affected human sensorimotor electrocortical dynamics using mobile high-density electroencephalography (EEG).

METHODS

To overcome limitations associated with motion and muscle artifact contamination in EEG recordings, we compared solutions for artifact removal using novel dual-layer EEG electrodes and alternative signal processing methods. Dual-layer EEG simultaneously recorded human electrocortical signals and isolated motion artifacts using pairs of mechanically coupled and electrically independent electrodes. For electrical muscle activity removal, we incorporated electromyographic (EMG) recordings from the neck into our mobile EEG data processing pipeline. We compared artifact removal methods during treadmill walking at four speeds (0.5, 1.0, 1.5, and 2.0 m/s).

RESULTS

Left and right sensorimotor alpha and beta spectral power increased in contralateral limb single support and push off, and decreased during contralateral limb swing at each speed. At faster walking speeds, sensorimotor spectral power fluctuations were less pronounced across the gait cycle with reduced alpha and beta power (p < 0.05) compared to slower speeds. Isolated noise recordings and neck EMG spectral power fluctuations matched gait events and showed broadband spectral power increases at faster speeds.

CONCLUSION AND SIGNIFICANCE

Dual-layer EEG enabled us to isolate changes in human sensorimotor electrocortical dynamics across walking speeds. A comparison of signal processing approaches revealed similar intrastride cortical fluctuations when applying common (e.g., artifact subspace reconstruction) and novel artifact rejection methods. Dual-layer EEG, however, allowed us to document and rule out residual artifacts, which exposed sensorimotor spectral power changes across gait speeds.

The beneficial aspects of spasticity in relation to ambulatory ability in mice with spinal cord injury

STUDY DESIGN

Experimental study with mice.

OBJECTIVES

Spasticity is a common complication after spinal cord injury (SCI) and has detrimental aspects, such as persistent pain and involuntary muscle spasms. This study aimed to assess the influence of antispastic therapy on locomotor function after SCI.

SETTING

University-based laboratory in Fukuoka, Japan.

METHODS

A mouse model of spasticity was developed by producing incomplete SCI at the 9th thoracic level. At 8 weeks after SCI, an antispastic drug, baclofen, was intraperitoneally administered to six injured and two sham-operated mice. The severity of spasticity was evaluated by the modified Ashworth scoring (MAS) system, and locomotor function was evaluated by the Basso-Beattie-Bresnahan (BBB) scale/Basso mouse score (BMS).

RESULTS

The administration of baclofen significantly improved spasticity in the SCI mice and the mean MAS decreased to from 6.2 to 2.8. However, at the same time, it significantly exacerbated the locomotor dysfunction of the SCI mice and the mean BMS decreased from 4.7 to 2.3. The time-course of the changes in locomotor function coincided with the time-course of the spasticity score. We also confirmed that the administration of baclofen was not associated with any changes in either locomotor function or spasticity of the sham-operated control mice.

CONCLUSIONS

Our results suggest that spasticity has a certain beneficial effect on ambulation ability. It is important to note that antispastic treatments may be associated with a risk of impairing the preserved function of chronic SCI patients.

Pioneers in CNS inhibition: 2. Charles Sherrington and John Eccles on inhibition in spinal and supraspinal structures

This article reviews the contributions of the English neurophysiologist, Charles Scott Sherrington [1857-1952], and his Australian PhD trainee and collaborator, John Carew Eccles [1903-1997], to the concept of central inhibition in the spinal cord and brain. Both were awarded Nobel Prizes; Sherrington in 1932 for "discoveries regarding the function of neurons," and Eccles in 1963 for "discoveries concerning the ionic mechanisms involved in excitation and inhibition in central portions of the nerve cell membrane." Both spoke about central inhibition at their Nobel Prize Award Ceremonies. The subsequent publications of their talks were entitled "Inhibition as a coordinative factor" and "The ionic mechanism of postsynaptic inhibition", respectively. Sherrington's work on central inhibition spanned 41 years (1893-1934), and for Eccles 49 years (1928-1977). Sherrington first studied central inhibition by observing hind limb muscle responses to electrical (peripheral nerve) and mechanical (muscle) stimulation. He used muscle length and force measurements until the early 1900s and electromyography in the late 1920s. Eccles used these techniques while working with Sherrington, but later employed extracellular microelectrode recording in the spinal cord followed in 1951 by intracellular recording from spinal motoneurons. This considerably advanced our understanding of central inhibition. Sherrington's health was poor during his retirement years but he nonetheless made a small number of largely humanities contributions up to 1951, one year before his death at the age of 94. In contrast, Eccles retained his health and vigor until 3 years before his death and published prolifically on many subjects during his 22 years of official retirement. His last neuroscience article appeared in 1994 when he was 91. Despite poor health he continued thinking about his life-long interest, the mind-brain problem, and was attempting to complete his autobiography in the last years of his life.

Central pattern generators in the brainstem and spinal cord: an overview of basic principles, similarities and differences

Central pattern generators (CPGs) are generally defined as networks of neurons capable of enabling the production of central commands, specifically controlling stereotyped, rhythmic motor behaviors. Several CPGs localized in brainstem and spinal cord areas have been shown to underlie the expression of complex behaviors such as deglutition, mastication, respiration, defecation, micturition, ejaculation, and locomotion. Their pivotal roles have clearly been demonstrated although their organization and cellular properties remain incompletely characterized. In recent years, insightful findings about CPGs have been made mainly because (1) several complementary animal models were developed; (2) these models enabled a wide variety of techniques to be used and, hence, a plethora of characteristics to be discovered; and (3) organizations, functions, and cell properties across all models and species studied thus far were generally found to be well-preserved phylogenetically. This article aims at providing an overview for non-experts of the most important findings made on CPGs in in vivo animal models, in vitro preparations from invertebrate and vertebrate species as well as in primates. Data about CPG functions, adaptation, organization, and cellular properties will be summarized with a special attention paid to the network for locomotion given its advanced level of characterization compared with some of the other CPGs. Similarities and differences between these networks will also be highlighted.

Feedback to the future: motor neuron contributions to central pattern generator function

Motor behaviors depend on neural signals in the brain. Regardless of where in the brain behavior patterns arise, the central nervous system sends projections to motor neurons, which in turn project to and control temporally appropriate muscle contractions; thus, motor neurons are traditionally considered the last relay from the central nervous system to muscles. However, in an array of species and motor systems, an accumulating body of evidence supports a more complex role of motor neurons in pattern generation. These studies suggest that motor neurons not only relay motor patterns to the periphery, but directly contribute to pattern generation by providing feedback to upstream circuitry. In spinal and hindbrain circuits in a variety of animals - including flies, worms, leeches, crustaceans, rodents, birds, fish, amphibians and mammals - studies have indicated a crucial role for motor neuron feedback in maintaining normal behavior patterns dictated by the activity of a central pattern generator. Hence, in this Review, we discuss literature examining the role of motor neuron feedback across many taxa and behaviors, and set out to determine the prevalence of motor neuron participation in motor circuits.

Resetting the Respiratory Rhythm with a Spinal Central Pattern Generator

There is evidence that a variety of central and afferent stimuli, including swallowing, can produce phase resetting in the respiratory rhythmicity. Also, there are reports about the intrinsic linkage between locomotion and respiration. However, little is known about the interaction between the central pattern generators (CPGs) for scratching and respiration. The present study aims to examine whether the activation of scratching CPG produces phase resetting of the respiratory rhythm. We employed decerebrate cats to apply brief tactile stimuli to the pinna during the inspiratory-expiratory transition. We observed that those stimuli to the pinna not eliciting fictive scratching did not reset the respiratory rhythm. However, when the pinna stimuli elicited fictive scratching, then the respiratory rhythm exhibited a significant phase resetting. We also found interneurons in the medulla oblongata exhibiting phase resetting related to scratching-CPG episodes. This second finding suggests that this type of resetting involves brainstem components of the respiratory CPG. These results shed new light on the resetting action from a spinal CPG on the respiratory rhythm.

Differential Cortical Control of Chest Wall Muscles During Pressure- and Volume-Related Expiratory Tasks and the Effects of Acute Expiratory Threshold Loading

We examined whether or not coherence between chest wall intercostal and oblique muscles changed as a function of lung volume excursion, alveolar pressure, and muscular demand. We also assessed the effects of acute expiratory threshold loading (ETL) on chest wall muscular control. A total of 15 healthy adults (7 males; average age = 28 years) completed maximum performance and ETL tasks. Chest wall surface electromyographic and kinematic recordings were made. Participants also performed a session of acute ETL. We showed that corticomuscular control of the chest wall varied as a function of lung volume excursion and muscular effort. Acute ETL had some effect on respiratory kinematics but not coherence.

Defining the Rhythmogenic Elements of Mammalian Breathing

Breathing's remarkable ability to adapt to changes in metabolic, environmental, and behavioral demands stems from a complex integration of its rhythm-generating network within the wider nervous system. Yet, this integration complicates identification of its specific rhythmogenic elements. Based on principles learned from smaller rhythmic networks of invertebrates, we define criteria that identify rhythmogenic elements of the mammalian breathing network and discuss how they interact to produce robust, dynamic breathing.

Breathing matters

Breathing is a well-described, vital and surprisingly complex behaviour, with behavioural and physiological outputs that are easy to directly measure. Key neural elements for generating breathing pattern are distinct, compact and form a network amenable to detailed interrogation, promising the imminent discovery of molecular, cellular, synaptic and network mechanisms that give rise to the behaviour. Coupled oscillatory microcircuits make up the rhythmic core of the breathing network. Primary among these is the preBötzinger Complex (preBötC), which is composed of excitatory rhythmogenic interneurons and excitatory and inhibitory pattern-forming interneurons that together produce the essential periodic drive for inspiration. The preBötC coordinates all phases of the breathing cycle, coordinates breathing with orofacial behaviours and strongly influences, and is influenced by, emotion and cognition. Here, we review progress towards cracking the inner workings of this vital core.

State-dependent rhythmogenesis and frequency control in a half-center locomotor CPG

The spinal locomotor central pattern generator (CPG) generates rhythmic activity with alternating flexion and extension phases. This rhythmic pattern is likely to result from inhibitory interactions between neural populations representing flexor and extensor half-centers. However, it is unclear whether the flexor-extensor CPG has a quasi-symmetric organization with both half-centers critically involved in rhythm generation, features an asymmetric organization with flexor-driven rhythmogenesis, or comprises a pair of intrinsically rhythmic half-centers. There are experimental data that support each of the above concepts but appear to be inconsistent with the others. In this theoretical/modeling study, we present and analyze a CPG model architecture that can operate in different regimes consistent with the above three concepts depending on conditions, which are defined by external excitatory drives to CPG half-centers. We show that control of frequency and phase durations within each regime depends on network dynamics, defined by the regime-dependent expression of the half-centers' intrinsic rhythmic capabilities and the operating phase transition mechanisms (escape vs. release). Our study suggests state dependency in locomotor CPG operation and proposes explanations for seemingly contradictory experimental data. NEW & NOTEWORTHY Our theoretical/modeling study focuses on the analysis of locomotor central pattern generators (CPGs) composed of conditionally bursting half-centers coupled with reciprocal inhibition and receiving independent external drives. We show that this CPG framework can operate in several regimes consistent with seemingly contradictory experimental data. In each regime, we study how intrinsic dynamics and phase-switching mechanisms control oscillation frequency and phase durations. Our results provide insights into the organization of spinal circuits controlling locomotion.

Different roles for inhibition in the rhythm-generating respiratory network

Unraveling the interplay of excitation and inhibition within rhythm-generating networks remains a fundamental issue in neuroscience. We use a biophysical model to investigate the different roles of local and long-range inhibition in the respiratory network, a key component of which is the pre-Bötzinger complex inspiratory microcircuit. Increasing inhibition within the microcircuit results in a limited number of out-of-phase neurons before rhythmicity and synchrony degenerate. Thus unstructured local inhibition is destabilizing and cannot support the generation of more than one rhythm. A two-phase rhythm requires restructuring the network into two microcircuits coupled by long-range inhibition in the manner of a half-center. In this context, inhibition leads to greater stability of the two out-of-phase rhythms. We support our computational results with in vitro recordings from mouse pre-Bötzinger complex. Partial excitation block leads to increased rhythmic variability, but this recovers after blockade of inhibition. Our results support the idea that local inhibition in the pre-Bötzinger complex is present to allow for descending control of synchrony or robustness to adverse conditions like hypoxia. We conclude that the balance of inhibition and excitation determines the stability of rhythmogenesis, but with opposite roles within and between areas. These different inhibitory roles may apply to a variety of rhythmic behaviors that emerge in widespread pattern-generating circuits of the nervous system.NEW & NOTEWORTHY The roles of inhibition within the pre-Bötzinger complex (preBötC) are a matter of debate. Using a combination of modeling and experiment, we demonstrate that inhibition affects synchrony, period variability, and overall frequency of the preBötC and coupled rhythmogenic networks. This work expands our understanding of ubiquitous motor and cognitive oscillatory networks.

Engagement of the Rat Hindlimb Motor Cortex across Natural Locomotor Behaviors

Contrary to cats and primates, cortical contribution to hindlimb locomotor movements is not critical in rats. However, the importance of the motor cortex to regain locomotion after neurological disorders in rats suggests that cortical engagement in hindlimb motor control may depend on the behavioral context. To investigate this possibility, we recorded whole-body kinematics, muscle synergies, and hindlimb motor cortex modulation in freely moving rats performing a range of natural locomotor procedures. We found that the activation of hindlimb motor cortex preceded gait initiation. During overground locomotion, the motor cortex exhibited consistent neuronal population responses that were synchronized with the spatiotemporal activation of hindlimb motoneurons. Behaviors requiring enhanced muscle activity or skilled paw placement correlated with substantial adjustment in neuronal population responses. In contrast, all rats exhibited a reduction of cortical activity during more automated behavior, such as stepping on a treadmill. Despite the facultative role of the motor cortex in the production of locomotion in rats, these results show that the encoding of hindlimb features in motor cortex dynamics is comparable in rats and cats. However, the extent of motor cortex modulations appears linked to the degree of volitional engagement and complexity of the task, reemphasizing the importance of goal-directed behaviors for motor control studies, rehabilitation, and neuroprosthetics.

SIGNIFICANCE STATEMENT

We mapped the neuronal population responses in the hindlimb motor cortex to hindlimb kinematics and hindlimb muscle synergies across a spectrum of natural locomotion behaviors. Robust task-specific neuronal population responses revealed that the rat motor cortex displays similar modulation as other mammals during locomotion. However, the reduced motor cortex activity during more automated behaviors suggests a relationship between the degree of engagement and task complexity. This relationship emphasizes the importance of the behavioral procedure to engage the motor cortex during motor control studies, gait rehabilitation, and locomotor neuroprosthetic developments in rats.

Spinal control of motor outputs by intrinsic and externally induced electric field potentials

Despite numerous studies on spinal neuronal systems, several issues regarding their role in motor behavior remain unresolved. One of these issues is how electric fields associated with the activity of spinal neurons influence the operation of spinal neuronal networks and how effects of these field potentials are combined with other means of modulating neuronal activity. Another closely related issue is how external electric field potentials affect spinal neurons and how they can be used for therapeutic purposes such as pain relief or recovery of motor functions by transspinal direct current stimulation. Nevertheless, progress in our understanding of the spinal effects of electric fields and their mechanisms has been made over the last years, and the aim of the present review is to summarize the recent findings in this field.

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.

What Is Being Trained? How Divergent Forms of Plasticity Compete To Shape Locomotor Recovery after Spinal Cord Injury

Spinal cord injury (SCI) is a devastating syndrome that produces dysfunction in motor and sensory systems, manifesting as chronic paralysis, sensory changes, and pain disorders. The multi-faceted and heterogeneous nature of SCI has made effective rehabilitative strategies challenging. Work over the last 40 years has aimed to overcome these obstacles by harnessing the intrinsic plasticity of the spinal cord to improve functional locomotor recovery. Intensive training after SCI facilitates lower extremity function and has shown promise as a tool for retraining the spinal cord by engaging innate locomotor circuitry in the lumbar cord. As new training paradigms evolve, the importance of appropriate afferent input has emerged as a requirement for adaptive plasticity. The integration of kinematic, sensory, and loading force information must be closely monitored and carefully manipulated to optimize training outcomes. Inappropriate peripheral input may produce lasting maladaptive sensory and motor effects, such as central pain and spasticity. Thus, it is important to closely consider the type of afferent input the injured spinal cord receives. Here we review preclinical and clinical input parameters fostering adaptive plasticity, as well as those producing maladaptive plasticity that may undermine neurorehabilitative efforts. We differentiate between passive (hindlimb unloading [HU], limb immobilization) and active (peripheral nociception) forms of aberrant input. Furthermore, we discuss the timing of initiating exposure to afferent input after SCI for promoting functional locomotor recovery. We conclude by presenting a candidate rapid synaptic mechanism for maladaptive plasticity after SCI, offering a pharmacological target for restoring the capacity for adaptive spinal plasticity in real time.

Microcircuits in respiratory rhythm generation: commonalities with other rhythm generating networks and evolutionary perspectives

Rhythmicity is critical for the generation of rhythmic behaviors and higher brain functions. This review discusses common mechanisms of rhythm generation, including the role of synaptic inhibition and excitation, with a focus on the mammalian respiratory network. This network generates three phases of breathing and is highly integrated with brain regions associated with numerous non-ventilatory behaviors. We hypothesize that during evolution multiple rhythmogenic microcircuits were recruited to accommodate the generation of each breathing phase. While these microcircuits relied primarily on excitatory mechanisms, synaptic inhibition became increasingly important to coordinate the different microcircuits and to integrate breathing into a rich behavioral repertoire that links breathing to sensory processing, arousal, and emotions as well as learning and memory.

Organization of flexor-extensor interactions in the mammalian spinal cord: insights from computational modelling

KEY POINTS

Alternation of flexor and extensor activity in the mammalian spinal cord is mediated by two classes of genetically identified inhibitory interneurons, V1 and V2b. The V1 interneurons are essential for high-speed locomotor activity. They secure flexor-extensor alternations in the intact cord but lose this function after hemisection, suggesting that they are activated by inputs from the contralateral side of the cord. The V2b interneurons are involved in flexor-extensor alternations in both intact cord and hemicords. We used a computational model of the spinal network, simulating the left and right rhythm-generating circuits interacting via several commissural pathways, and extended this model by incorporating V1 and V2b neuron populations involved in flexor-extensor interactions on each cord side. The model reproduces multiple experimental data on selective silencing and activation of V1 and/or V2b neurons and proposes the organization of their connectivity providing flexor-extensor alternation in the spinal cord.

ABSTRACT

Alternating flexor and extensor activity represents the fundamental property underlying many motor behaviours including locomotion. During locomotion this alternation appears to arise in rhythm-generating circuits and transpires at all levels of the spinal cord including motoneurons. Recent studies in vitro and in vivo have shown that flexor-extensor alternation during locomotion involves two classes of genetically identified, inhibitory interneurons: V1 and V2b. Particularly, in the isolated mouse spinal cord, abrogation of neurotransmission derived by both V1 and V2b interneurons resulted in flexor-extensor synchronization, whereas selective inactivation of only one of these neuron types did not abolish flexor-extensor alternation. After hemisection, inactivation of only V2b interneurons led to the flexor-extensor synchronization, while inactivation of V1 interneurons did not affect flexor-extensor alternation. Moreover, optogenetic activation of V2b interneurons suppressed extensor-related activity, while similar activation of V1 interneurons suppressed both flexor and extensor oscillations. Here, we address these issues using the previously published computational model of spinal circuitry simulating bilateral interactions between left and right rhythm-generating circuits. In the present study, we incorporate V1 and V2b neuron populations on both sides of the cord to make them critically involved in flexor-extensor interactions. The model reproduces multiple experimental data on the effects of hemisection and selective silencing or activation of V1 and V2b neurons and suggests connectivity profiles of these neurons and their specific roles in left-right (V1) and flexor-extensor (both V2b and V1) interactions in the spinal cord that can be tested experimentally.

Decoding the organization of spinal circuits that control locomotion

Unravelling the functional operation of neuronal networks and linking cellular activity to specific behavioural outcomes are among the biggest challenges in neuroscience. In this broad field of research, substantial progress has been made in studies of the spinal networks that control locomotion. Through united efforts using electrophysiological and molecular genetic network approaches and behavioural studies in phylogenetically diverse experimental models, the organization of locomotor networks has begun to be decoded. The emergent themes from this research are that the locomotor networks have a modular organization with distinct transmitter and molecular codes and that their organization is reconfigured with changes to the speed of locomotion or changes in gait.