Neural mechanisms generating locomotion studied in mammalian brain stem-spinal cord in vitro

The neural control system for generation of locomotion is an important system for analysis of neural mechanisms underlying complex motor acts. In these studies, a novel experimental model using neonatal rat brain stem and spinal cord in vitro was developed for investigation of the locomotor system in mammals. The in vitro brain stem and spinal cord system was shown to retain functional circuitry for locomotor command generation, motor pattern generation, and sensorimotor integration. This system was exploited to investigate neurochemical mechanisms involved in neurogenesis of locomotion. Evidence was obtained for peptidergic and gamma-amino-butyric acid-mediated mechanisms in brain-stem circuits generating locomotor commands. Cholinergic, dopaminergic, and excitatory amino acid-mediated mechanisms were shown to activate spinal cord circuits for locomotor pattern generation. Endogenous N-methyl-D-aspartic acid receptors in spinal networks were found to play a central role in the generation of locomotion. The chemically induced patterns of motor activity and rhythmic membrane potential oscillations of spinal motoneurons were characteristic of those during locomotion in other mammals in vivo. The in vitro brain stem and spinal cord model provides a versatile and powerful experimental system with potentially broad application for investigation of diverse aspects of the neurobiology of mammalian motor control systems.     

A novel neural substrate for the transformation of olfactory inputs into motor output

It is widely recognized that animals respond to odors by generating or modulating specific motor behaviors. These reactions are important for daily activities, reproduction, and survival. In the sea lamprey, mating occurs after ovulated females are attracted to spawning sites by male sex pheromones. The ubiquity and reliability of olfactory-motor behavioral responses in vertebrates suggest tight coupling between the olfactory system and brain areas controlling movements. However, the circuitry and the underlying cellular neural mechanisms remain largely unknown. Using lamprey brain preparations, and electrophysiology, calcium imaging, and tract tracing experiments, we describe the neural substrate responsible for transforming an olfactory input into a locomotor output. We found that olfactory stimulation with naturally occurring odors and pheromones induced large excitatory responses in reticulospinal cells, the command neurons for locomotion. We have also identified the anatomy and physiology of this circuit. The olfactory input was relayed in the medial part of the olfactory bulb, in the posterior tuberculum, in the mesencephalic locomotor region, to finally reach reticulospinal cells in the hindbrain. Activation of this olfactory-motor pathway generated rhythmic ventral root discharges and swimming movements. Our study bridges the gap between behavior and cellular neural mechanisms in vertebrates, identifying a specific subsystem within the CNS, dedicated to producing motor responses to olfactory inputs.     

Genetic identification of spinal interneurons that coordinate left-right locomotor activity necessary for walking movements

The sequential stepping of left and right limbs is a fundamental motor behavior that underlies walking movements. This relatively simple locomotor behavior is generated by the rhythmic activity of motor neurons under the control of spinal neural networks known as central pattern generators (CPGs) that comprise multiple interneuron cell types. Little, however, is known about the identity and contribution of defined interneuronal populations to mammalian locomotor behaviors. We show a discrete subset of commissural spinal interneurons, whose fate is controlled by the activity of the homeobox gene Dbx1, has a critical role in controlling the left-right alternation of motor neurons innervating hindlimb muscles. Dbx1 mutant mice lacking these ventral interneurons exhibit an increased incidence of cobursting between left and right flexor/extensor motor neurons during drug-induced locomotion. Together, these findings identify Dbx1-dependent interneurons as key components of the spinal locomotor circuits that control stepping movements in mammals.     

Ontogeny of a Simple Locomotor System: Role of the Periphery in the Development of Central Nervous Circuitry

The development of locomotor systems in the lobster Homarusamericanus is described. The tail—flip escape responseis fully developed when the larvae hatch, and occurs withoutthe participation of giant fibers. The abdominal swimmeretsare undifferentiated at hatching, but are fully developed twoto three weeks later when the animals molt to the fourth larvalstage. Forward locomotion in the pelagic larvae is achievedusing thoracic swimming appendages until the fourth larval stage,when these degenerate and the swimmerets assume the locomotorrole. The hypothesis that peripheral structures specify the centralnervous connections of motoneurons during ontogeny was testedin the swimmeret system. Presumptive swimmeret appendages, includingprospective muscle and sense organs, were extirpated prior totheir differentiation in newly hatched larvae. The correspondingswimmeret motoneurons nevertheless grew and formed normal centralconnections, as evidenced by the appearance of normal patternsof rhythmic locomotor discharge and normal reflexes at the usualtime. Moreover, swimmeret motoneurons retained normal patternsof motor output even when the regeneration of their target appendageswas prevented for as long as two months. Therefore, the formationof normal motor output patterns during ontogeny is not dependentupon feedback from differentiated target muscle nor from senseorgans which normally monitor the results of the motor activity.     

Contribution of stretch reflexes to locomotor control: a modeling study

It is known that the springlike properties of muscles provide automatic load compensation during weight bearing. How crucial is sensory control of the motor output given these basic properties of the locomotor system? To address this question, a neuromuscular model was used to test two hypotheses. (1) Stretch reflexes are too weak and too delayed to contribute significantly to weight-bearing. (2) The important contributions of sensory input involve state-dependent processing. We constructed a two-legged planar locomotor model with 9 segments, driven by 12 musculotendon actuators with Hill-type force-velocity and monotonic force-length properties. Electromyographic (EMG) profiles of the simulated muscle groups during slow level walking served as actuator activation functions. Spindle Ia and tendon organ Ib sensory inputs were represented by transfer functions with a latency of 35 ms, contributing 30% to the net EMG profile and gated to be active only when the receptor-bearing muscles were contracting. Locomotor stability was assessed by parametric variations of actuator maximum forces during locomotion in open-loop ("deafferented") trials and in trials with feedback control based on either sensory-evoked stretch reflexes or finite-state rules. We arrived at the following conclusions. (1) In the absence of sensory control, the intrinsic stiffness of limb muscles driven by a stereotyped rhythmical pattern can produce surprisingly stable gait. (2) When the level of central activity is low, the contribution of stretch reflexes to load compensation can be crucial. However, when central activity provides adequate load compensation, the contribution of stretch reflexes is less significant. (3) Finite-state control can greatly extend the adaptive capability of the locomotor system.     

Xenopus laevis: an ideal experimental model for studying the developmental dynamics of neural network assembly and sensory-motor computations

The amphibian Xenopus laevis represents a highly amenable model system for exploring the ontogeny of central neural networks, the functional establishment of sensory-motor transformations, and the generation of effective motor commands for complex behaviors. Specifically, the ability to employ a range of semi-intact and isolated preparations for in vitro morphophysiological experimentation has provided new insights into the developmental and integrative processes associated with the generation of locomotory behavior during changing life styles. In vitro electrophysiological studies have begun to explore the functional assembly, disassembly and dynamic plasticity of spinal pattern generating circuits as Xenopus undergoes the developmental switch from larval tail-based swimming to adult limb-based locomotion. Major advances have also been made in understanding the developmental onset of multisensory signal processing for reactive gaze and posture stabilizing reflexes during self-motion. Additionally, recent evidence from semi-intact animal and isolated CNS experiments has provided compelling evidence that in Xenopus tadpoles, predictive feed-forward signaling from the spinal locomotor pattern generator are engaged in minimizing visual disturbances during tail-based swimming. This new concept questions the traditional view of retinal image stabilization that in vertebrates has been exclusively attributed to sensory-motor transformations of body/head motion-detecting signals. Moreover, changes in visuomotor demands associated with the developmental transition in propulsive strategy from tail- to limb-based locomotion during metamorphosis presumably necessitates corresponding adaptive alterations in the intrinsic spinoextraocular coupling mechanism. Consequently, Xenopus provides a unique opportunity to address basic questions on the developmental dynamics of neural network assembly and sensory-motor computations for vertebrate motor behavior in general.     

Artificial neural network model for the generation of muscle activation patterns for human locomotion.

Skilled locomotor behaviour requires information from various levels within the central nervous system (CNS). Mathematical models have permitted researchers to simulate various mechanisms in order to understand the organization of the locomotor control system. While it is difficult to adequately characterize the numerous inputs to the locomotor control system, an alternative strategy may be to use a kinematic movement plan to represent the complex inputs to the locomotor control system based on the possibility that the CNS may plan movements at a kinematic level. We propose the use of artificial neural network (ANN) models to represent the transformation of a kinematic plan into the necessary motor patterns. Essentially, kinematic representation of the actual limb movement was used as the input to an ANN model which generated the EMG activity of 8 muscles of the lower limb and trunk. Data from a wide variety of gait conditions was necessary to develop a robust model that could accommodate various environmental conditions encountered during everyday activity. A total of 120 walking strides representing normal walking and ten conditions where the normal gait was modified in terms of cadence, stride length, stance width or required foot clearance. The final network was assessed on its ability to predict the EMG activity on individual walking trials as well as its ability to represent the general activation pattern of a particular gait condition. The predicted EMG patterns closely matched those recorded experimentally, exhibiting the appropriate magnitude and temporal phasing required for each modification. Only 2 of the 96 muscle/gait conditions had RMS errors above 0.10, only 5 muscle/gait conditions exhibited correlations below 0.80 (most were above 0.90) and only 25 muscle/gait conditions deviated outside the normal range of muscle activity for more than 25% of the gait cycle. These results indicate the ability of single network ANNs to represent the transformation between a kinematic movement plan and the necessary muscle activations for normal steady state locomotion but they were also able to generate muscle activation patterns for conditions requiring changes in walking speed, foot placement and foot clearance. The abilities of this type of network have implications towards both the fundamental understanding of the control of locomotion and practical realizations of artificial control systems for use in rehabilitation medicine.     

Flexibility in the patterning and control of axial locomotor networks in lamprey

In lower vertebrates, locomotor burst generators for axial muscles generally produce unitary bursts that alternate between the two sides of the body. In lamprey, a lower vertebrate, locomotor activity in the axial ventral roots of the isolated spinal cord can exhibit flexibility in the timings of bursts to dorsally-located myotomal muscle fibers versus ventrally-located myotomal muscle fibers. These episodes of decreased synchrony can occur spontaneously, especially in the rostral spinal cord where the propagating body waves of swimming originate. Application of serotonin, an endogenous spinal neurotransmitter known to presynaptically inhibit excitatory synapses in lamprey, can promote decreased synchrony of dorsal-ventral bursting. These observations suggest the possible existence of dorsal and ventral locomotor networks with modifiable coupling strength between them. Intracellular recordings of motoneurons during locomotor activity provide some support for this model. Pairs of motoneurons innervating myotomal muscle fibers of similar ipsilateral dorsoventral location tend to have higher correlations of fast synaptic activity during fictive locomotion than do pairs of motoneurons innervating myotomes of different ipsilateral dorsoventral locations, suggesting their control by different populations of premotor interneurons. Further, these different motoneuron pools receive different patterns of excitatory and inhibitory inputs from individual reticulospinal neurons, conveyed in part by different sets of premotor interneurons. Perhaps, then, the locomotor network of the lamprey is not simply a unitary burst generator on each side of the spinal cord that activates all ipsilateral body muscles simultaneously. Instead, the burst generator on each side may comprise at least two coupled burst generators, one controlling motoneurons innervating dorsal body muscles and one controlling motoneurons innervating ventral body muscles. The coupling strength between these two ipsilateral burst generators may be modifiable and weakening when greater swimming maneuverability is required. Variable coupling of intrasegmental burst generators in the lamprey may be a precursor to the variable coupling of burst generators observed in the control of locomotion in the joints of limbed vertebrates.     

Asymmetries in postural control and locomotion in chimpanzees (Pan troglodytes)

Posture and locomotion are two of the most primitive and basic motor manifestations of an organism's behavior. Although the restrictions they impose on other motor functions are evident, few studies have considered the possibility of asymmetries in these behaviors in human and nonhuman primates, and how they might impact other asymmetries at higher functional levels. The aim of the current study was to explore in a group of 10 chimpanzees at the Madrid Zoo-Aquarium the degree of asymmetry in four behaviors related to locomotion (walking, ascending, descending, and brachiating) and four behaviors associated with posture (sitting, lying, hanging, and changing postures). Few subjects showed individual preferences, but significant trends in the group for some of the behaviors were found, including right-hand use when initiating quadruped walking, and left-hand use when descending and hanging. Some significant correlations also emerged: a negative one between walking and descending, and a positive one between walking and brachiating and between sitting and changing postures. No correlations were found between locomotor and postural modes. Although we cannot make generalizations on the population level at this time, these findings highlight the importance of considering postural and locomotion factors when studying motor asymmetries in primates.     

Peptidergic neuromodulation of the lumbar locomotor network in the neonatal rat spinal cord

It is now well established that a dynamic balance of neurotransmitters and neuromodulators finely influence the output of neuronal networks and subsequent behaviors. In the present study, to further understand the modulatory processes that control locomotor behavior, we investigated the action of 11 neuropeptides, chosen among the various peptide subfamilies, on the lumbar neuronal network in the in vitro neonatal rat spinal cord preparation. Peptides were bath-applied alone, in combination with N-methyl-D,L-aspartate (NMA) or with the classical 'locomotor cocktail' of NMA and serotonin. Using these different experimental paradigms, we show that each peptide can neuromodulate the lumbar locomotor network and that peptides exhibit different neuromodulatory profiles and potencies even within the same family. Only vasopressin, oxytocin, bombesin and thyrotropin releasing hormone triggered tonic or non-organized rhythmic activities when bath-applied alone. All the neuropeptides modulated NMA induced activity and/ or ongoing sequences of fictive locomotion to varying degrees. These results suggest that neuropeptides play an important role in the control of the neural network for locomotion in the neonatal rat. Their various profiles of action may account in part for the great flexibility of motor behaviors.     

Functional regeneration and recovery of locomotor activity in spinally transected lamprey.

Spinally transected lamprey recovery locomotor function within 3-6 weeks, and recovery is due, in part, to functional regeneration of neural pathways in the central nervous system (CNS). Our data demonstrate for the first time in the lamprey that descending axons arising from brainstem command neurons can functionally regenerate and restore locomotor initiation below a healed spinal transection site. Immediately after behavioral recovery (3-6 weeks) the locomotor pattern was incomplete but returned to normal during the remainder of the recovery period (6-40 weeks). Initially, the extent of regeneration of descending axons was limited but increased to at least 30-50 mm at recovery times of 24-40 weeks. Regenerated giant Muller axons do not contribute significantly to recovery of locomotor function; rather, regenerated axons of smaller reticulospinal neurons appear to restore locomotor initiation. The restoration of locomotor coordination across a spinal lesion is dependent on two mechanisms: regeneration of spinal coordinating neurons and mechanosensory inputs. Comparisons are made to spinal cord regeneration in other lower vertebrates and to the relative lack of CNS regeneration and behavioral recovery in higher vertebrates.     

A Pair of Dopamine Neurons Target the D1-Like Dopamine Receptor DopR in the Central Complex to Promote Ethanol-Stimulated Locomotion in Drosophila

Dopamine is a mediator of the stimulant properties of drugs of abuse, including ethanol, in mammals and in the fruit fly Drosophila. The neural substrates for the stimulant actions of ethanol in flies are not known. We show that a subset of dopamine neurons and their targets, through the action of the D1-like dopamine receptor DopR, promote locomotor activation in response to acute ethanol exposure. A bilateral pair of dopaminergic neurons in the fly brain mediates the enhanced locomotor activity induced by ethanol exposure, and promotes locomotion when directly activated. These neurons project to the central complex ellipsoid body, a structure implicated in regulating motor behaviors. Ellipsoid body neurons are required for ethanol-induced locomotor activity and they express DopR. Elimination of DopR blunts the locomotor activating effects of ethanol, and this behavior can be restored by selective expression of DopR in the ellipsoid body. These data tie the activity of defined dopamine neurons to D1-like DopR-expressing neurons to form a neural circuit that governs acute responding to ethanol.     

运动过程的网络逻辑——从离子通道到动物行为

为了揭示神经网络在脊椎动物运动中所行使的内在功能,作者开发了七鳃鳗这种低等脊椎动物模型。在这套系统中,不仅可以了解到运动模式生成网络以及激活此网络的命令系统,同时还可以在运动中研究方向控制系统和变向控制系统。七鳃鳗的神经系统有较少的神经元,而且运动行为中的不同运动模式可以由分离的神经系统所引发。模式生成神经网络包括同侧的谷氨酸能中间神经元和对侧的抑制性甘氨酸能中间神经元。网络中的突触连接、细胞膜特性和神经递质都也已经被鉴定。运动是由脑干区域的网状脊髓神经元所引起,而这些神经元又是被问脑和中脑分离的一些运动命令神经元群所控制。因此,运动行为最初是由这两个“运动核心”所启动。而这两个运动核心被基底神经节调控,基底神经节即时地做出判断是否允许下游的运动程序启动。在静止情况下基底神经节的输出核团维持对下游不同运动核心的抑制作用,反之则去除抑制活化运动核心。纹状体和苍白球被认为是这个运动抉择系统的主要部件。根据“霍奇金一贺胥黎”模型神经元开发了这套网络模型,不同的细胞具有各自相应的不同亚型的钠、钾、钙离子通道和钙依赖的钾通道。每个模型神经元拥有86个不同区域模块以及其对应的生物学功能,例如频率控制、超极化等等。然后根据已有实验证据,利用突触将不同的模型神经元相连。而系统中的10000个神经元大致和生物学网络上的细胞数量相当。突触数量为760000。突触类型有AMPA、NMDA、glycine型。有了这样大规模的模型,不仅可以模拟肌节与肌节之间的神经网络,还可以模拟到由基底神经节开始的行为起始部分。此外,这些网络模拟还被用于一个神经机械学模型来模拟包含有推进和方向控制部分的真实运动。     

Interaction between descending input and thoracic reflexes for joint coordination in cockroach: I. Descending influence on thoracic sensory reflexes

Tethered cockroaches turn from unilateral antennal contact using asymmetrical movements of mesothoracic (T2) legs (Mu and Ritzmann in J Comp Physiol A 191:1037–1054, 2005). During the turn, the leg on the inside of the turn (the inside T2 leg) has distinctly different motor patterns from those in straight walking. One possible neural mechanism for the transformation from walking to inside leg turning could be that the descending commands alter a few critical reflexes that start a cascade of physical changes in leg movement or posture, leading to further alterations. This hypothesis has two implications: first, the descending activities must be able to influence thoracic reflexes. Second, one should be able to initiate the turning motor pattern without descending signals by mimicking a point farther down in the reflex cascade. We addressed the first implication in this paper by experiments on chordotonal organ reflexes. The activity of depressor muscle (Ds) and slow extensor tibia muscle (SETi) was excited and inhibited by stretching and relaxing the femoral chordotonal organ. However, the Ds responses were altered after eliminating the descending activity, while the SETi responses remain similar. The inhibition to Ds activity by stretching the coxal chordotonal organ was also altered after eliminating the descending activity.     

Mechanisms for lateral turns in lamprey in response to descending unilateral commands: a modeling study

 Straight locomotion in the lamprey is, at the segmental level, characterized by alternating bursts of motor activity with equal duration and spike frequency on the left and the right sides of the body. Lateral turns are characterized by three main changes in this pattern: (1) in the turn cycle, the spike frequency, burst duration, and burst proportion (burst duration/cycle duration) increase on the turning side; (2) the cycle duration increases in both the turn cycle and the succeeding cycle; and (3) in the cycle succeeding the turn cycle, the burst duration increases on the non-turning side (rebound). We investigated mechanisms for the generation of turns in single-segment models of the lamprey locomotor spinal network. Activation of crossing inhibitory neurons proved a sufficient mechanism to explain all three changes in the locomotor rhythm during a fictive turn. Increased activation of these cells inhibits the activity of the opposite side during the prolonged burst of the turn cycle, and slows down the locomotor rhythm. Secondly, this activation of the crossing inhibitory neurons is accompanied by an increased calcium influx into the cells. This gives a suppressed activity on the turning side and a contralateral rebound after the turn, through activation of calcium-dependent potassium channels. Received: 28 June 2000 / Accepted for publication: 10 May 2001     

CTCF regulates ataxin-7 expression through promotion of a convergently transcribed, antisense noncoding RNA

Neural networks in the spinal cord control two basic features of locomotor movements: rhythm generation and pattern generation. Rhythm generation is generally considered to be dependent on glutamatergic excitatory neurons. Pattern generation involves neural circuits controlling left-right alternation, which has been described in great detail, and flexor-extensor alternation, which remains poorly understood. Here, we use a mouse model in which glutamatergic neurotransmission has been ablated in the locomotor region of the spinal cord. The isolated in?vitro spinal cord from these mice produces locomotor-like activity-when stimulated with neuroactive substances-with prominent flexor-extensor alternation. Under these conditions, unlike in control mice, networks of inhibitory interneurons generate the rhythmic activity. In the absence of glutamatergic synaptic transmission, the flexor-extensor alternation appears to be generated by Ia inhibitory interneurons, which mediate reciprocal inhibition from muscle proprioceptors to antagonist motor neurons. Our study defines a minimal inhibitory network that is needed to produce flexor-extensor alternation during locomotion.     

mGluR5 Ablation in Cortical Glutamatergic Neurons Increases Novelty-Induced Locomotion

The group I metabotropic glutamate receptor 5 (mGluR5) has been implicated in the pathology of various neurological disorders including schizophrenia, ADHD, and autism. mGluR5-dependent synaptic plasticity has been described at a variety of neural connections and its signaling has been implicated in several behaviors. These behaviors include locomotor reactivity to novel environment, sensorimotor gating, anxiety, and cognition. mGluR5 is expressed in glutamatergic neurons, inhibitory neurons, and glia in various brain regions. In this study, we show that deleting mGluR5 expression only in principal cortical neurons leads to defective cannabinoid receptor 1 (CB1R) dependent synaptic plasticity in the prefrontal cortex. These cortical glutamatergic mGluR5 knockout mice exhibit increased novelty-induced locomotion, and their locomotion can be further enhanced by treatment with the psychostimulant methylphenidate. Despite a modest reduction in repetitive behaviors, cortical glutamatergic mGluR5 knockout mice are normal in sensorimotor gating, anxiety, motor balance/learning and fear conditioning behaviors. These results show that mGluR5 signaling in cortical glutamatergic neurons is required for precisely modulating locomotor reactivity to a novel environment but not for sensorimotor gating, anxiety, motor coordination, several forms of learning or social interactions.     

Locomotion induced by epidural stimulation in a decerebrated cat after spinal cord injury

The effect of partial and complete spinal cord transection (Th7–Th8) on locomotor activity evoked in decerebrated cats by electrical epidural stimulation (segment L5, 80–100 μA, 0.5 ms at 5 Hz) has been investigated. Transection of dorsal columns did not substantially influence the locomotion. Disruption of the ventral spinal quadrant resulted in deterioration and instability of the locomotor rhythm. Injury to lateral or medial descending motor systems led to redistribution of the tone in antagonist muscles. Locomotion could be evoked by epidural stimulation within 20 h after complete transection of the spinal cord. The restoration of polysynaptic components in EMG responses correlated with recovery of the stepping function. The data obtained confirm that initiation of locomotion under epidural stimulation is caused by direct action on intraspinal systems responsible for locomotor regulation. With intact or partially injured spinal cord, this effect is under the influence of supraspinal motor systems correcting and stabilizing the evoked locomotor pattern.