The neural network underlying locomotion in lamprey--synaptic and cellular mechanisms

Subthalamic and pallidal neurons are modulated during externally cued movements in Parkinson's disease

External sensory cues can reduce freezing of gait in people with Parkinson's disease (PD), yet the role of the basal ganglia in these movements is unclear. We used microelectrode recordings to examine modulations in single unit (SU) and oscillatory local field potentials (LFP) during auditory-cued rhythmic pedaling movements of the feet. We tested five blocks of increasing cue frequencies (1 Hz, 1.5 Hz, 2 Hz, 2.5 Hz, and 3 Hz) in 24 people with PD undergoing deep brain stimulation surgery of the subthalamic nucleus (STN) or globus pallidus internus (GPi). Single unit firing and beta band LFPs (13-30 Hz) in response to movement onsets or cue onsets were examined. We found that the timing accuracy of foot pedaling decreased with faster cue frequencies. Increasing cue frequencies also attenuated firing rates in both STN and GPi neurons. Peak beta power in the GPi and STN showed different responses to the task. GPi beta power showed persistent suppression with fast cues and phasic modulation with slow cues. STN beta power showed enhanced beta synchronization following movement. STN beta power also correlated with rate of pedaling. Overall, we showed task-related responses in the GPi and STN during auditory-cued movements with differential roles in sensory and motor control. The results suggest a role for both input and output basal ganglia nuclei in auditory rhythmic pacing of gait-like movements in PD.

Hydrodynamics of Biomimetic Marine Propulsion and Trends in Computational Simulations

The aim of the present paper is to provide the state of the works in the field of hydrodynamics and computational simulations to analyze biomimetic marine propulsors. Over the last years, many researchers postulated that some fish movements are more efficient and maneuverable than traditional rotary propellers, and the most relevant marine propulsors which mimic fishes are shown in the present work. Taking into account the complexity and cost of some experimental setups, numerical models offer an efficient, cheap, and fast alternative tool to analyze biomimetic marine propulsors. Besides, numerical models provide information that cannot be obtained using experimental techniques. Since the literature about trends in computational simulations is still scarce, this paper also recalls the hydrodynamics of the swimming modes occurring in fish and summarizes the more relevant lines of investigation of computational models.

A novel method for quantifying periodicity and time delay in dynamic neural networks using unstable subaction potential threshold depolarizations

Techniques to identify and correlate the propagation of electrical signals (like action potentials) along neural networks are well described, using multisite recordings. In these cases, the waveform of action potentials is usually relatively stable and discriminating relevant electrical signals straightforward. However, problems can arise when attempting to identify and correlate the propagation of signals when their waveforms are unstable (e.g., fluctuations in amplitude or time course). This makes correlation of the degree of synchronization and time lag between propagating electrical events across two or more recording sites problematic. Here, we present novel techniques for the determination of the periodicity of electrical signals at individual sites. When recording from two independent sites, we present novel analytical techniques for joint determination of periodicity and time delay. The techniques presented exploit properties of the cross-correlation function, rather than utilizing the time lag at which the cross-correlation function is maximized. The approach allows determination of directionality of the spread of excitation along a neural network based on measurements of the time delay between recording sites. This new method is particularly applicable to analysis of signals in other biological systems that have unstable characteristics in waveform that show dynamic variability.NEW & NOTEWORTHY The determination of frequency(s) at which two sources are synchronized, and relative time delay between them, is a fundamental problem for a wide a range of signal-processing applications. In this methodology paper, we present novel procedures for periodicity estimation for single time series and joint periodicity and time delay estimation for two time series. The methods use properties of the cross-correlation function rather than the cross-correlation function explicitly.

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.

Behavioral Role of the Reciprocal Inhibition between a Pair of Mauthner Cells during Fast Escapes in Zebrafish

During many behaviors in vertebrates, the CNS generates asymmetric activities between the left and right sides to produce asymmetric body movements. For asymmetrical activations of the CNS, reciprocal inhibition between the left and right sides is believed to play a key role. However, the complexity of the CNS makes it difficult to identify the reciprocal inhibition circuits at the level of individual cells and the contribution of each neuron to the asymmetric activity. Using larval zebrafish, we examined this issue by investigating reciprocal inhibition circuits between a pair of Mauthner (M) cells, giant reticulospinal neurons that trigger fast escapes. Previous studies have shown that a class of excitatory neurons, called cranial relay neurons, is involved in the reciprocal inhibition pathway between the M cells. Using transgenic fish, in which two of the cranial relay neurons (Ta1 and Ta2) expressed GFP, we showed that Ta1 and Ta2 constitute major parts of the pathway. In larvae in which Ta1/Ta2 were laser-ablated, the amplitude of the reciprocal IPSPs dropped to less than one-third. Calcium imaging and electrophysiological recording showed that the occurrence probability of bilateral M-cell activation upon sound/vibration stimuli was greatly increased in the Ta1/Ta2-ablated larvae. Behavioral experiments revealed that the Ta1/Ta2 ablation resulted in shallower body bends during sound/vibration-evoked escapes, which is consistent with the observation that increased occurrence of bilateral M-cell activation impaired escape performance. Our study revealed major components of the reciprocal inhibition circuits in the M cell system and the behavioral importance of the circuits.SIGNIFICANCE STATEMENT Reciprocal inhibition between the left and right side of the CNS is considered imperative for producing asymmetric movements in animals. It has been difficult, however, to identify the circuits at the individual cell level and their role in behavior. Here, we address this problem by examining the reciprocal inhibition circuits of the hindbrain Mauthner (M) cell system in larval zebrafish. We determined that two paired interneurons play a critical role in the reciprocal inhibition between the paired M cells and that the reciprocal inhibition prevents bilateral firing of the M cells and is thus necessary for the full body bend during M cell-initiated escape. Further, we discussed the cooperation of multiple reciprocal inhibitions working in the hindbrain and spinal cord to ensure high-performance escapes.

GABAergic inhibition of leg motoneurons is required for normal walking behavior in freely moving Drosophila

Inhibition is an important feature of the neuronal circuit, and in walking, it aids in controlling coordinated movement of legs, leg segments, and joints. Recent studies in Drosophila report the role of premotor inhibitory interneurons in regulation of larval locomotion. However, in adult walking, the identity and function of premotor interneurons are poorly understood. Here, we use genetic methods for targeted knockdown of inhibitory neurotransmitter receptors in leg motoneurons, combined with automated video recording methods we have developed for quantitative analysis of fly leg movements and walking parameters, to reveal the resulting slower walking speed and defects in walking parameters. Our results indicate that GABAergic premotor inhibition to leg motoneurons is required to control the normal walking behavior in adult Drosophila.

Walking is a complex rhythmic locomotor behavior generated by sequential and periodical contraction of muscles essential for coordinated control of movements of legs and leg joints. Studies of walking in vertebrates and invertebrates have revealed that premotor neural circuitry generates a basic rhythmic pattern that is sculpted by sensory feedback and ultimately controls the amplitude and phase of the motor output to leg muscles. However, the identity and functional roles of the premotor interneurons that directly control leg motoneuron activity are poorly understood. Here we take advantage of the powerful genetic methodology available in Drosophila to investigate the role of premotor inhibition in walking by genetically suppressing inhibitory input to leg motoneurons. For this, we have developed an algorithm for automated analysis of leg motion to characterize the walking parameters of wild-type flies from high-speed video recordings. Further, we use genetic reagents for targeted RNAi knockdown of inhibitory neurotransmitter receptors in leg motoneurons together with quantitative analysis of resulting changes in leg movement parameters in freely walking Drosophila. Our findings indicate that targeted down-regulation of the GABA_A receptor Rdl (Resistance to Dieldrin) in leg motoneurons results in a dramatic reduction of walking speed and step length without the loss of general leg coordination during locomotion. Genetically restricting the knockdown to the adult stage and subsets of motoneurons yields qualitatively identical results. Taken together, these findings identify GABAergic premotor inhibition of motoneurons as an important determinant of correctly coordinated leg movements and speed of walking in freely behaving Drosophila.

A method for detailed movement pattern analysis of tadpole startle response

Prolonged space flight, specifically microgravity, presents a problem for space exploration. Animal models with altered connections of the vestibular ear, and thus altered gravity sensation, would allow the examination of the effects of microgravity and how various countermeasures can establish normal function. We describe an experimental apparatus to monitor the effects of ear manipulations to generate asymmetric gravity input on the tadpole escape response. To perform the movement pattern analysis, an imaging apparatus was developed that uses a high-speed camera to obtain time-resolved, high-resolution images of tadpole movements. Movements were recorded in a temperature-controlled test chamber following mechanical stimulation with a solenoid actuator, to elicit a C-start response. Temperature within the test cell was controlled with a recirculating water bath. Xenopus laevis embryos were obtained using a standard fertilization technique. Tadpole response to a controlled perturbation was recorded in unprecedented detail and the approach was validated by describing the distinct differences in response between normal and one-eared tadpoles. The experimental apparatus and methods form an important element of a rigorous investigation into the response of the tadpole vestibular system to mechanical and biochemical manipulations, and can ultimately contribute to improved understanding of the effects of altered gravity perception on humans.

Neonatal seizure automatism and human inborn pattern of quadrupedal locomotion

Seizures in newborns do not always show a clear electro-clinical correlation. The real epileptic nature of some stereotyped rhythmic movements, included in the 'subtle seizures' and considered as brainstem release phenomena, is still debated. We report a brain injured newborn, who displayed several episodes of repetitive limb movements. The ictal EEG discharge, during one of these episodes, was associated with a motor pattern modification, which was endowed with quadrupedal locomotion kinematic features. This might represent an indirect evidence of cervical and lumbar Central Pattern Generators interconnection with in-phase coordination between diagonal limbs since the first hours of life in humans.

Review : Recovery from Spinal Cord Injury: New Approaches

Damage to the spinal cord has devastating consequences because injury induces neuronal death, and the severed neural pathway does not regenerate. One of the major challenges in this field is to rescue injured neurons and to stimulate the regrowth of severed axons. An equally important task is the re-establishment of precise connections by regenerating axons with their targets and remyelinating the regenerated axons so that they can propagate action potentials. In our view, the major hurdles that must be overcome to restore function after spinal cord injury can be viewed as a recapitulation of steps normally taken during development. Recent basic research advances provide cautious optimism that therapies will be available for acute and chronic spinal cord injury. The Neuroscientist 1:321-327, 1995

A group of segmental premotor interneurons regulates the speed of axial locomotion in Drosophila larvae

BACKGROUND

Animals control the speed of motion to meet behavioral demands. Yet, the underlying neuronal mechanisms remain poorly understood. Here we show that a class of segmentally arrayed local interneurons (period-positive median segmental interneurons, or PMSIs) regulates the speed of peristaltic locomotion in Drosophila larvae.

RESULTS

PMSIs formed glutamatergic synapses on motor neurons and, when optogenetically activated, inhibited motor activity, indicating that they are inhibitory premotor interneurons. Calcium imaging showed that PMSIs are rhythmically active during peristalsis with a short time delay in relation to motor neurons. Optogenetic silencing of these neurons elongated the duration of motor bursting and greatly reduced the speed of larval locomotion.

CONCLUSIONS

Our results suggest that PMSIs control the speed of axial locomotion by limiting, via inhibition, the duration of motor outputs in each segment. Similar mechanisms are found in the regulation of mammalian limb locomotion, suggesting that common strategies may be used to control the speed of animal movements in a diversity of species.

A bidirectional brain-machine interface algorithm that approximates arbitrary force-fields

We examine bidirectional brain-machine interfaces that control external devices in a closed loop by decoding motor cortical activity to command the device and by encoding the state of the device by delivering electrical stimuli to sensory areas. Although it is possible to design this artificial sensory-motor interaction while maintaining two independent channels of communication, here we propose a rule that closes the loop between flows of sensory and motor information in a way that approximates a desired dynamical policy expressed as a field of forces acting upon the controlled external device. We previously developed a first implementation of this approach based on linear decoding of neural activity recorded from the motor cortex into a set of forces (a force field) applied to a point mass, and on encoding of position of the point mass into patterns of electrical stimuli delivered to somatosensory areas. However, this previous algorithm had the limitation that it only worked in situations when the position-to-force map to be implemented is invertible. Here we overcome this limitation by developing a new non-linear form of the bidirectional interface that can approximate a virtually unlimited family of continuous fields. The new algorithm bases both the encoding of position information and the decoding of motor cortical activity on an explicit map between spike trains and the state space of the device computed with Multi-Dimensional-Scaling. We present a detailed computational analysis of the performance of the interface and a validation of its robustness by using synthetic neural responses in a simulated sensory-motor loop.

Shaping the dynamics of a bidirectional neural interface

Progress in decoding neural signals has enabled the development of interfaces that translate cortical brain activities into commands for operating robotic arms and other devices. The electrical stimulation of sensory areas provides a means to create artificial sensory information about the state of a device. Taken together, neural activity recording and microstimulation techniques allow us to embed a portion of the central nervous system within a closed-loop system, whose behavior emerges from the combined dynamical properties of its neural and artificial components. In this study we asked if it is possible to concurrently regulate this bidirectional brain-machine interaction so as to shape a desired dynamical behavior of the combined system. To this end, we followed a well-known biological pathway. In vertebrates, the communications between brain and limb mechanics are mediated by the spinal cord, which combines brain instructions with sensory information and organizes coordinated patterns of muscle forces driving the limbs along dynamically stable trajectories. We report the creation and testing of the first neural interface that emulates this sensory-motor interaction. The interface organizes a bidirectional communication between sensory and motor areas of the brain of anaesthetized rats and an external dynamical object with programmable properties. The system includes (a) a motor interface decoding signals from a motor cortical area, and (b) a sensory interface encoding the state of the external object into electrical stimuli to a somatosensory area. The interactions between brain activities and the state of the external object generate a family of trajectories converging upon a selected equilibrium point from arbitrary starting locations. Thus, the bidirectional interface establishes the possibility to specify not only a particular movement trajectory but an entire family of motions, which includes the prescribed reactions to unexpected perturbations.

Feed forward and feedback control for over-ground locomotion in anaesthetized cats

The biological central pattern generator (CPG) integrates open and closed loop control to produce over-ground walking. The goal of this study was to develop a physiologically based algorithm capable of mimicking the biological system to control multiple joints in the lower extremities for producing over-ground walking. The algorithm used state-based models of the step cycle each of which produced different stimulation patterns. Two configurations were implemented to restore over-ground walking in five adult anaesthetized cats using intramuscular stimulation (IMS) of the main hip, knee and ankle flexor and extensor muscles in the hind limbs. An open loop controller relied only on intrinsic timing while a hybrid-CPG controller added sensory feedback from force plates (representing limb loading), and accelerometers and gyroscopes (representing limb position). Stimulation applied to hind limb muscles caused extension or flexion in the hips, knees and ankles. A total of 113 walking trials were obtained across all experiments. Of these, 74 were successful in which the cats traversed 75% of the 3.5 m over-ground walkway. In these trials, the average peak step length decreased from 24.9 ± 8.4 to 21.8 ± 7.5 (normalized units) and the median number of steps per trial increased from 7 (Q1 = 6, Q3 = 9) to 9 (8, 11) with the hybrid-CPG controller. Moreover, within these trials, the hybrid-CPG controller produced more successful steps (step length ≤ 20 cm; ground reaction force ≥ 12.5% body weight) than the open loop controller: 372 of 544 steps (68%) versus 65 of 134 steps (49%), respectively. This supports our previous preliminary findings, and affirms that physiologically based hybrid-CPG approaches produce more successful stepping than open loop controllers. The algorithm provides the foundation for a neural prosthetic controller and a framework to implement more detailed control of locomotion in the future.

Neuronal control of swimming behavior: comparison of vertebrate and invertebrate model systems

Swimming movements in the leech and lamprey are highly analogous, and lack homology. Thus, similarities in mechanisms must arise from convergent evolution rather than from common ancestry. Despite over 40 years of parallel investigations into this annelid and primitive vertebrate, a close comparison of the approaches and results of this research is lacking. The present review evaluates the neural mechanisms underlying swimming in these two animals and describes the many similarities that provide intriguing examples of convergent evolution. Specifically, we discuss swim initiation, maintenance and termination, isolated nervous system preparations, neural-circuitry, central oscillators, intersegmental coupling, phase lags, cycle periods and sensory feedback. Comparative studies between species highlight mechanisms that optimize behavior and allow us a broader understanding of nervous system function.

Synchronized bursts of miniature inhibitory postsynaptic currents

Spike-independent miniature postsynaptic currents are generally stochastic and are therefore not thought to mediate information relay in neuronal circuits. However, we recorded endogenous bursts of IPSCs in hypothalamic magnocellular neurones in the presence of TTX, which implicated a coordinated mechanism of spike-independent GABA release. IPSC bursts were identical in the absence of TTX, although the burst incidence increased 5-fold, indicating that IPSC bursts were composed of miniature IPSCs (mIPSCs), and that the probability of burst generation increased with action potential activity. IPSC bursts required extracellular calcium, although they were not dependent on calcium influx through voltage-gated calcium channels or on calcium mobilization from intracellular stores. Current injections simulating IPSC bursts were capable of triggering and terminating action potential trains. In 25% of dual recordings, a subset of IPSC bursts were highly synchronized in onset in pairs of magnocellular neurones. Synchronized IPSC bursts displayed properties that were consistent with simultaneous release at GABA synapses shared between pairs of postsynaptic magnocellular neurones. Synchronized bursts of inhibitory synaptic inputs represent a novel mechanism that may contribute to the action potential burst generation, termination and synchronization responsible for pulsatile hormone release from neuroendocrine cells.

Interruption of pacemaker signals is mediated by GABAergic inhibition of the pacemaker nucleus in the African electric fish Gymnarchus niloticus

The wave-type African weakly electric fish Gymnarchus niloticus produces electric organ discharges (EODs) from an electric organ in the tail that is driven by a pacemaker complex in the medulla, which consists of a pacemaker nucleus, two lateral relay nuclei and a medial relay nucleus. The prepacemaker nucleus (PPn) in the area of the dorsal posterior nucleus of the thalamus projects exclusively to the pacemaker nucleus and is responsible for EOD interruption behavior. The goal of the present study is to test the existence of inhibition of the pacemaker nucleus by the PPn. Immunohistochemical results showed clear anti-GABA immunoreactive labeling of fibers and terminals in the pacemaker nucleus, but no apparent anti-glycine immunoreactivity anywhere in the pacemaker complex. GABA injection into the pacemaker nucleus could induce EOD interruptions that are comparable to the interruptions induced by glutamate injection into the PPn. Application of the GABA(A) receptor blocker bicuculline methiodide reversibly eliminated the effects of stimulation of the PPn. Thus the EOD interruption behavior in Gymnarchus is mediated through GABAergic inhibition of the pacemaker nucleus by the PPn.

Projections of primary afferent fibers to last-order premotor interneurons in the lumbar spinal cord of rats

It is well established that last-order premotor interneurons in the spinal cord have crucial importance in the integration of activities generated by the spinal motor apparatus, sensory information and volleys arising from higher motor centers, indicating that they play a substantial role in spinal motor functions. Despite extensive studies, synaptic input systems of these neurons have not been investigated in detail up to now with morphological approaches. On this basis, the present experiments were aimed at the visualization of possible contacts between primary afferents and last-order premotor interneurons in the lumbar spinal cord of rats using double label neural tracing methods in light microscopy. The findings show that terminal puncta of primary afferents do establish indeed appositions on last-order premotor interneurons. From the quantitative point of view, these appositions occur, however, in limited numbers. The study also shows that last-order premotor interneurons contacted by primary afferents tend to be concentrated at the segmental level of the innervated motoneurons, and are evenly distributed along the mediolateral extent of laminae V-VI and in the dorsal portion of lamina VII.

Complexities and uncertainties of neuronal network function

The nervous system generates behaviours through the activity in groups of neurons assembled into networks. Understanding these networks is thus essential to our understanding of nervous system function. Understanding a network requires information on its component cells, their interactions and their functional properties. Few networks come close to providing complete information on these aspects. However, even if complete information were available it would still only provide limited insight into network function. This is because the functional and structural properties of a network are not fixed but are plastic and can change over time. The number of interacting network components, their (variable) functional properties, and various plasticity mechanisms endows networks with considerable flexibility, but these features inevitably complicate network analyses. This review will initially discuss the general approaches and problems of network analyses. It will then examine the success of these analyses in a model spinal cord locomotor network in the lamprey, to determine to what extent in this relatively simple vertebrate system it is possible to claim detailed understanding of network function and plasticity.

The effects of varying the timing of inputs on a neural oscillator

The gastric mill network of the stomatogastric ganglion of the crab Cancer borealis is comprised of a set of neurons that require modulatory input from outside the stomatogastric ganglion and input from the pyloric network of the animal in order to oscillate. Here we study how the frequency of the gastric mill network is determined when it receives rhythmic input from two different sources but where the timing of these inputs may differ. We find that over a certain range of the time difference one of the two rhythmic inputs plays no role what so ever in determining the network frequency, while in another range, both inputs work together to determine the frequency. The existence and stability of periodic solutions to model sets of equations are obtained analytically using geometric singular perturbation theory. The results are validated through numerical simulations. Comparisons to experiments are also presented.

Rhythmic motor activity evoked by NMDA in the spinal zebrafish larva

We have examined the localization and activity of the neural circuitry that generates swimming behavior in developing zebrafish that were spinalized to isolate the spinal cord from descending brain inputs. We found that addition of the excitatory amino acid agonist N-methyl-d-aspartate (NMDA) to spinalized zebrafish at 3 days in development induced repeating episodes of rhythmic tail beating activity reminiscent of slow swimming behavior. The neural correlate of this activity, monitored by extracellular recording comprised repeating episodes of rhythmic, rostrocaudally progressing peripheral nerve discharges that alternated between the two sides of the body. Motoneuron recordings revealed an activity pattern comprising a slow oscillatory and a fast synaptic component that was consistent with fictive swimming behavior. Pharmacological and voltage-clamp analysis implicated glycine and glutamate in generation of motoneuron activity. Contralateral alternation of motor activity was disrupted with strychnine, indicating a role for glycine in coordinating left-right alternation during NMDA-induced locomotion. At embryonic stages, while rhythmic synaptic activity patterns could still be evoked in motoneurons, they were typically lower in frequency. Kinematic recordings revealed that prior to 3 days in development, NMDA was unable to reliably generate rhythmic tail beating behavior. We conclude that NMDA induces episodes of rhythmic motor activity in spinalized developing zebrafish that can be monitored physiologically in paralyzed preparations. Therefore as for other vertebrates, the zebrafish central pattern generator is intrinsic to the spinal cord and can operate in isolation provided a tonic source of excitation is given.

Calcium imaging of network function in the developing spinal cord

We have used calcium imaging to visualize the spatiotemporal organization of activity generated by in vitro spinal cord preparations of the developing chick embryo and the neonatal mouse. During each episode of spontaneous activity, we found that chick spinal neurons were activated rhythmically and synchronously throughout the transverse extent of the spinal cord. At the onset of a spontaneous episode, optical activity originated in the ventrolateral part of the cord. Back-labeling of spinal interneurons with calcium dyes suggested that this ventrolateral initiation was mediated by activation of a class of interneurons, located dorsomedial to the motor nucleus, that receive direct monosynaptic input from motoneurons. Studies of locomotor-like activity in the anterior lumbar segments of the neonatal mouse cord revealed the existence of a rostrocaudal wave in the oscillatory component of each cycle of rhythmic motoneuron activity. This finding raises the possibility that the activation of mammalian motoneurons during locomotion may share some of the same rostrocaudally organized mechanisms that evolved to control swimming in fishes.

Activation of NMDA receptors is required for the initiation and maintenance of walking-like activity in the mudpuppy (Necturus Maculatus)

We hypothesized that blocking the activation of N-methyl-D-aspartate (NMDA) receptors prevents the initiation of walking-like activity and abolishes the ongoing rhythmic activity in the spinal cord-forelimb preparation from the mudpuppy. Robust walking-like movements of the limb and rhythmic alternating elbow flexor-extensor EMG pattern characteristic of walking were elicited when continuous perfusion of the spinal cord with solution containing D-glutamate. The frequency of the walking-like activity was dose-dependent on the concentration of D-glutamate in the bath over a range of 0.2 to 0.9 mmol/L. Elevation of potassium concentrations failed to induce walking-like activity. Application of the selective antagonist 2-amino-5-phosphonovalerate (AP-5) produced dose-dependent block of the initiation and maintenance of walking-like activity induced by D-glutamate. Complete block of the activity was achieved when the concentration of AP-5 reached 20 micromol/L. Furthermore, application of L-701,324 (a selective antagonist of the strychnine-insensitive glycine site of NMDA receptor) (1-10 micromol/L) also resulted in complete block of the walking-like activity. In contrast, application of the non-NMDA receptor antagonist 6-cyno-7-nitroquinoxaline-2,3-dione (CNQX) (1-50 micromol/L) induced a dose-dependent inhibition of the burst frequency but failed to result in a complete block. Only at concentration as high as 100 micromol/L, did CNQX cause complete block of the rhythmic activity, presumably through nonspecific action on the strychnine-insensitive glycine site of NMDA receptors. These results suggest that activation of NMDA receptors is required for the initiation and maintenance of walking-like activity. Operation of non-NMDA receptors plays a powerful role in the modulation of the walking-like activity in the mudpuppy.

Fictive swimming motor patterns in wild type and mutant larval zebrafish

Larval zebrafish provide a unique model for investigating the mechanisms involved in generating rhythmic patterns of behavior, such as swimming, due to the array of techniques available including genetics, optical imaging, and conventional electrophysiology. Because electrophysiological and imaging studies of rhythmic motor behaviors in paralyzed preparations depend on the ability to monitor the central motor pattern, we developed a fictive preparation in which the activity of axial motor neurons was monitored using extracellular recordings from peripheral nerves. We examined spontaneous and light induced fictive motor patterns in wild type and mutant larval zebrafish (4-6 days post-fertilization) paralyzed with curare. All spontaneous and light-induced preparations produced alternation of motor activity from side-to-side (mean contralateral phase = 50.7 +/- 7.0%; mean burst frequency = 35.6 +/- 4.7 Hz) and a progression of activity from head-to-tail (mean ipsilateral rostrocaudal delay = 0.8 +/- 0.5 ms per segment), consistent with lateral undulation and forward propulsion during swimming, respectively. The basic properties of the motor pattern were similar in spontaneous and light-induced swimming. This fictive preparation can be used in combination with conventional electrophysiological and imaging methods to investigate normal circuit function as well as to elucidate functional deficits in mutant lines. Toward this end, we show that two accordion class mutants, accordion and bandoneon, have alternating activity on opposite sides of the body, contradicting the hypothesis that their deficit results from the absence of the reciprocal glycinergic inhibition that is typically found in the spinal cord of swimming vertebrates.

Effects of limb exercise after spinal cord injury on motor neuron dendrite structure

An integration center subserving locomotor leg movements resides in the upper lumbar spinal cord. If this neuronal network is preserved after a spinal cord injury, it is possible to stimulate this circuitry to initiate and promote walking. The several effective approaches (electrical stimulation, pharmacologic agents, physical therapy training programs) may all share a common modus operandi of altering synaptic activity within segmental spinal cord. To understand the neural substrate for the use-dependent behavioral improvement, we studied the dendritic architecture of spinal motor neurons. In the first experiment, we compared three groups of animals: animals with an intact spinal cord, animals that had a complete spinal cord transection (SCT) and animals with SCT who engaged in a daily exercise program of actively moving paralyzed hindlimbs through the motions of walking. When compared with animals with an intact spinal cord, the motor neurons from animals with SCT displayed marked atrophy, with loss of dendritic membrane and elimination of branching throughout the visible tree within transverse tissue slices. None of these regressive changes were found in the motor neurons from SCT animals that underwent exercise. In a second experiment, we inquired whether exercise of animals with an intact spinal cord influenced dendrite structure. Increased exercise had very modest effects on dendrite morphology, indicating an upper limit of use-dependent dendrite growth. Our findings suggest that the dendritic tree of motor neurons deprived of descending influences is rapidly pruned, and this finding is not observed in motor neurons after SCT if hindlimbs are exercised. The functional benefits of exercise after SCT injury may be subserved, in part, by stabilizing or remodeling the dendritic tree of motor neurons below the injury site.

Steps during the development of the zebrafish locomotor network

This review summarizes recent data from our lab concerning the development of motor activities in the developing zebrafish. The zebrafish is a leading model for studies of vertebrate development because one can obtain a large number of transparent, externally and rapidly developing embryos with motor behaviors that are easy to assess (e.g. for mutagenic screens). The emergence of embryonic motility was studied behaviorally and at the cellular level. The embryonic behaviors appear sequentially and include an early, transient period of spontaneous, alternating tail coilings, followed by responses to touch, and swimming. Patch clamp recording in vivo revealed that an electrically coupled network of a subset of spinal neurons generates spontaneous tail coiling, whereas a chemical (glutamatergic and glycinergic) synaptic drive underlies touch responses and swimming and requires input from the hindbrain. Swimming becomes sustained in larvae once serotonergic neuromodulatory effects are integrated. We end with a brief overview of the genetic tools available for the study of the molecular determinants implicated in locomotor network development in the zebrafish. Combining genetic, behavioral and cellular experimental approaches will advance our understanding of the general principles of locomotor network assembly and function.

Developmental changes in rhythmic spinal neuronal activity in the rat fetus

In the developing rat spinal cord, formation and differentiation of the central pattern generator for locomotion occur during the prenatal period. Early on, excitatory synaptic transmission mediated by glycine receptors plays a leading role for rhythmogenesis, at a later stage, followed by glutamate-receptor-mediated synaptic transmission becoming dominant. The maturation of inhibitory circuitry in the spinal cord, mediated largely by glycinergic synapses, is crucial for the generation of alternating activity between left/right limbs and flexor/extensor muscles. Formation of left/right alternation is presumably due to developmental changes in the properties of the postsynaptic neurons, themselves, whereas flexor/extensor alternation requires the additional emergence of inhibitory synaptic functions in the spinal cord.

Ventral neurons in the anterior nerve cord of amphioxus larvae. I. An inventory of cell types and synaptic patterns

Serial sections were used to map the ventrally positioned neurons of the anterior nerve cord of a 12.5-day amphioxus larva from the infundibular region to the end of somite 2. Synaptic patterns reveal five categories of descending pathways, four of which are associated with the ventral compartment (VC) motoneurons responsible for escape swimming. 1) Pre-, para-, and postinfundibular (tegmental) neurons with large varicosities and mixed vesicle populations provide both synaptic and paracrine input to various components of the tegmental neuropile and primary motor center. Four categories of these neurons are distinguished on the basis of their vesicles. 2) Multiple anterior sensory pathways converge on the large paired neurons (LPNs) located near the junction of somites 1 and 2. LPN synaptic output is almost exclusively contralateral. This, together with the evidence for cross-innervation between the third pair of LPNs, is consistent with the latter acting as locomotory pacemakers. 3) Axons from several classes of tegmental neurons converge in the paraxial region on each side of the cord where they form distinct tracts, the upper paraxial bundles. The right bundle is larger than the left, which suggests a role during early development when myotome contractions are biased to one side. 4) Fibers in the ventral tracts from ipsilateral projection neurons, sensory neurons, and additional ascending fibers synapse repeatedly with VC motoneurons. This may be how the overall level of excitation of the latter is controlled so as to modulate their response to pacemaker input. The fifth pathway consists of fibers involved in controlling the dorsal compartment (DC) motoneurons responsible for slow swimming, which are largely isolated from inputs to the VC locomotory system. The ventral neurons of the primary motor center form a more or less continuous file on either side of the floor plate, with certain cell types showing a tendency to cluster. There are, however, few obvious patterns of the kind expected if development were controlled by a rigid, lineage-based mechanism. The evolutionary implications of the involvement of a midbrain-level pacemaker in controlling larval swimming in amphioxus is discussed.

Development of the locomotor network in zebrafish

The zebrafish is a leading model for studies of vertebrate development and genetics. Its embryonic motor behaviors are easy to assess (e.g. for mutagenic screens), the embryos develop rapidly (hatching as larvae at 2 days) and are transparent, permitting calcium imaging and patch clamp recording in vivo. We review primarily the recent advances in understanding the cellular basis for the development of motor activities in the developing zebrafish. The motor activities are generated largely in the spinal cord and hindbrain. In the embryo these segmented structures possess a relatively small number of repeating sets of identifiable neurons. Many types of neurons as well as the two types of muscle cells have been classified based on their morphologies. Some of the molecular signals for cellular differentiation have been identified recently and mutations affecting cell development have been isolated. Embryonic motor behaviors appear in sequence and consist of an early period of transient spontaneous coiling contractions, followed by the emergence of twitching responses to touch, and later by the ability to swim. Coiling contractions are generated by an electrically coupled network of a subset of spinal neurons whereas a chemical (glutamatergic and glycinergic) synaptic drive underlies touch responses and swimming. Swimming becomes sustained in larvae once the neuromodulatory serotonergic system develops. These results indicate many similarities between developing zebrafish and other vertebrates in the properties of the synaptic drive underlying locomotion. Therefore, the zebrafish is a useful preparation for gaining new insights into the development of the neural control of vertebrate locomotion. As the types of neurons, transmitters, receptors and channels used in the locomotor network are being defined, this opens the possibility of combining cellular neurophysiology with forward and reverse molecular genetics to understand the principles of locomotor network assembly and function.

Locomotor-like activity generated by the neonatal mouse spinal cord

This report describes locomotor-like activity generated by the neonatal mouse spinal cord in vitro. We demonstrate that locomotor-like activity can be produced either spontaneously or by a train of stimuli applied to the dorsal roots or in the presence of bath-applied drugs. Calcium imaging of the motoneuron activity generated by a train of dorsal root stimuli revealed a rostrocaudally propagating component of the optical signal in the anterior lumbar (L1-L3) and in the caudal segments (S1-S4). We hypothesize that this spatio-temporal pattern arises from a rostrocaudal gradient of excitability in the relevant segments. Our experiments suggest that left/right reciprocal inhibition and NMDA-mediated oscillations are not essential mechanisms underlying rhythmogenesis in the neonatal mouse cord. Finally, our data are discussed in the context of other models of locomotion in lower and higher vertebrates.

The dorsal compartment locomotory control system in amphioxus larvae

Amphioxus myotomes consist of separate sets of superficial and deep muscle fibers, each with its own innervation, that are thought to be responsible for slow swimming and escape behavior, respectively. Tracings from serial EM sections of the anterior nerve cord in the larva show that the motoneurons and premotor interneurons controlling the superficial fibers (the dorsal compartment, or DC pathway) are linked by specialized junctions of a previously undescribed type, referred to here as juxta-reticular (JR) junctions for the characteristic presence of a cisterna of endoplasmic reticulum on each side. JR junctions link the DC motoneurons with each other, with the largest of the anterior paired neurons (LPN3s) and with one class of ipsilateral projection neurons (IPNs), but occur nowhere else. Because of the paucity of synaptic input to the DC system, larval behavior can only be explained if the JR junctions act as functional links between cells. An analysis of the pattern of cell contacts also suggests that the LPN3s are probably pacemakers for both slow and fast locomotion, but act through junctions in the former case and conventional synapses in the latter. The only major synaptic input to the DC system identified in somites 1 and 2 was from four neurons located in the cerebral vesicle, referred to here as Type 2 preinfundibular projection neurons (PPN2s). They have unusually large varicosities, arranged in series, that make periodic contacts with the DC motoneurons. More caudally, the DC motoneurons receive additional input via similar large varicosities from the receptor cells of the first dorsal ocellus, located in somite 5. The overall circuitry of the locomotory control system suggests that the PPN2s may be instrumental in sustaining slow swimming, whereas mechanical stimulation, especially of the rostrum, preferentially activates the fast mode.

Central pattern generation of locomotion: a review of the evidence

Neural networks in the spinal cord, referred to as "central pattern generators" (CPGs), are capable of producing rhythmic movements, such as swimming, walking, and hopping, even when isolated from the brain and sensory inputs. This article reviews the evidence for CPGs governing locomotion and addresses other factors, including supraspinal, sensory, and neuromodulatory influences, that interact with CPGs to shape the final motor output. Supraspinal inputs play a major role not only in initiating locomotion but also in adapting the locomotor pattern to environmental and motivational conditions. Sensory afferents involved in muscle and cutaneous reflexes have important regulatory functions in preserving balance and ensuring stable phase transitions in the locomotor cycle. Neuromodulators evoke changes in cellular and synaptic properties of CPG neurons, conferring flexibility to CPG circuits. Briefly addressed is the interaction of CPG networks to produce intersegmental coordination for locomotion. Evidence for CPGs in humans is reviewed, and although a comprehensive clinical review is not an objective, examples are provided of animal and human studies that apply knowledge of CPG mechanisms to improve locomotion. The final section deals with future directions in CPG research.

In vivo imaging of zebrafish reveals differences in the spinal networks for escape and swimming movements

Most studies of spinal interneurons in vertebrate motor circuits have focused on the activity of interneurons in a single motor behavior. As a result, relatively little is known about the extent to which particular classes of spinal interneurons participate in different behaviors. Similarities between the morphology and connections of interneurons activated in swimming and escape movements in different fish and amphibians led to the hypothesis that spinal interneurons might be shared by these behaviors. To test this hypothesis, we took advantage of the optical transparency of zebrafish larvae and developed a new preparation in which we could use confocal calcium imaging to monitor the activity of individual identified interneurons noninvasively, while we simultaneously filmed the movements of the fish with a high-speed digital camera. With this approach, we could directly examine the involvement of individual interneurons in different motor behaviors. Our work revealed unexpected differences in the interneurons activated in swimming and escape behaviors. The observations lead to predictions of different behavioral roles for particular classes of spinal interneurons that can eventually be tested directly in zebrafish by using laser ablations or mutant lines with interneuronal deficits.

Recovery of locomotor function in adult paraplegic frogs by inductive lability in the distal isolated spinal cord neural networks

We postulated (Krishnan, 1991) that in a spinal cord transected adult paraplegic mammal locomotor functions can be revived if polyneuronal innervation is reinduced in the paralyzed hind limb muscles. This procedure destabilizes the neural networks and induces new synaptic growth in the distal isolated cord. In this pilot project we tested the hypothesis in cord-transected adult paraplegic frogs. Polyneuronal innervation was reinduced by crushing the sciatic nerve in the right upper thigh. Left limb sciatic nerve was not crushed and served as control. Another group of adult frogs had only cord transection without nerve crush. Five to seven weeks postnerve crush, full powered flexion-extension movements in the hip and knee joints appeared in the right hind limb and were used for swimming and surface progression. Movements gradually declined over the next weeks, which in some animals was seen preserved even beyond 120 days. Paraplegic frogs without nerve crush did not show any recovery of locomotor function. Interestingly, the uncrushed contralateral limb also produced transient, weak locomotor-like movements. This lasted for 4 to 6 days and waned out completely thereafter. These results validate our hypothesis on methods to generate new synaptic sprouts and reconnections to redrive the locomotor system. We had recommended earlier that destabilization procedure should be included as an essential component in treatment strategies for spinal cord injury repair for effective relinking of the severed cord ends.

Two distinct oscillatory states determined by the NMDA receptor in rat inferior olive

1. The effects of N-methyl-D-aspartate (NMDA) receptor activation and blockade on subthreshold membrane potential oscillations of inferior olivary neurones were studied in brainstem slices from 12- to 21-day-old rats. 2. Dizocilpine (MK-801), a non-competitive NMDA antagonist, at 1-45 microM abolished spontaneous subthreshold oscillations, without affecting membrane potential, input resistance, or the low-threshold calcium current, I(T). Ketamine (100 microM), a non-competitive NMDA antagonist, and L-689,560 (20 microM), an antagonist at the glycine site of the NMDA receptor, also abolished the oscillations, while the competitive non-NMDA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 20-50 microM) had no effect. 3. NMDA (100 microM) induced 4.1 Hz subthreshold oscillations and reversibly depolarized olivary neurones by 13.7 mV. In contrast, 10 microM alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and 20 microM kainic acid depolarized the membrane equivalently but did not induce oscillations. 4. Both NMDA-induced and spontaneous subthreshold oscillations were unaffected by 1 microM tetrodotoxin and were prevented by substituting extracellular calcium with cobalt. 5. Removing magnesium from the perfusate did not affect spontaneous subthreshold oscillations but did prevent NMDA-induced oscillations. 6. NMDA-induced oscillations were resistant to 50 microM mibefradil, an I(T) blocker, in contrast to spontaneous oscillations. Both oscillations were inhibited by 20 microM nifedipine, an L-type calcium channel antagonist, and 200 nM omega-agatoxin IVA, a P-type calcium channel blocker. Bay K 8644 (10 microM), an L-type Ca(2+) agonist, significantly enhanced the amplitude of both spontaneous and NMDA-induced oscillations. 7. The data indicate that NMDA receptor activation induces olivary neurones to manifest high amplitude membrane potential oscillations in part mediated by L- and P- but not T-type calcium currents. Moreover, the data demonstrate that NMDA receptor currents are necessary for generation of spontaneous subthreshold oscillations in the inferior olive.

Structural and physiological properties of connections between individual reticulospinal axons and lumbar motoneurons of the frog

Although the direct, monosynaptic influence of brainstem projections onto motoneurons is well-known, detailed morphological studies on the synaptic contact systems and a correlation with their functional properties are largely lacking. In this work, 43 pairs, each formed by a reticulospinal fiber contacting a lumbar motoneuron, were identified and studied electrophysiologically. Four of these were successfully labeled intracellularly with horseradish peroxidase (HRP) or neurobiotin and reconstructed using a computer-assisted camera lucida with high resolution. The mean amplitude of excitatory post-synaptic potentials (EPSPs) recorded in these four pairs varied from 100 to 730 microV, spanning most of the range obtained for all pairs (70-1,200 microV; mean +/- SD: 400 +/- 250 microV). Between two and four collaterals of reticulospinal axons established 4-19 close appositions with a labeled motoneuron. Mean distance from the origin of each collateral to any bouton on that collateral was 566-817 microm. A presynaptic action potential must pass 11 branch points on average to reach it. Similarly, the boutons presumably contacting motoneurons were on average 558-624 microm (9-11 branch points) from the origin of the collateral. The distributions of diameters of all boutons and those making putative contacts with stained motoneurons were very similar. The dendritic surface of stained motoneurons was symmetrically distributed along the rostrocaudal axis with more than half the surface being more than 500 microm from the soma. However, the contacts from reticulospinal axons were concentrated ventromedially, 262-356 microm (range of average values for four connections) from the motoneuron soma, in some instances on very proximal dendritic segments. Thus, the location and size of putative contacts in relation to axonal collaterals was not distinguishable from location and size of other boutons, but they occupied specific positions on dendrites of lumbar motoneurons. The number of contacts formed by a reticulospinal axon on a motoneuron in a particular location could be described as the product of the available dendritic surface and the total number of presynaptic boutons in this region. Compartmental models of the reconstructed motoneurons were created, and currents with the time course of an alpha function were injected at the sites of these putative contacts. Despite the restricted volume occupied by contacts from a single fiber, a high variability of their contributions to somatic EPSPs owing to electrotonic attenuation was shown: The coefficient of variation of quantal responses was estimated to be between 60% and 120%, comparable to the variability of the path distance between contacts and soma (50-90%).

Pattern generation in non-limb moving segments of the mammalian spinal cord

The ability of mammalian spinal cords to generate rhythmic motor patterns has been traditionally studied in hindlimb innervating segments of the spinal cord. The rhythmogenic capacity of these segments decreases substantially in the rostrocaudal direction so that the caudal lumbar segments are virtually incapable of producing the rhythm. Our recent studies of the sacrococcygeal segments of the neonatal rat spinal cord showed that these non-limb innervating segments have an intrinsic rhythmogenic capacity that is used to elicit rhythmic tail movements. The high viability of the sacrococcygeal segments, the specific behavior produced by them, and their simple functional organization, makes the isolated sacrocaudal network a new promising model for studies of neural automaticity in mammals. The present work summarizes the current knowledge on sacrococcygeal rhythmicity and discusses its functional implication.

Formation of the central pattern generator for locomotion in the rat and mouse

It is well known that in the neonatal rat spinal cord preparation, alternating rhythmic bursts in the left and right ventral roots in a given lumbar segment can be induced by bath-application of N-methyl-D-aspartate or 5-hydroxytryptamine. Alternation between L2 and L5 ventral roots on the same side, representing the activity of flexor and extensor muscles, respectively, can be observed as well. In the prenatal period in the rat, alternation between the left and right ventral roots is established between embryonic day (E) 16.5 and E18.5. The alternation between the L2 and L5 ventral roots emerges at E20.5. Recent findings show that locomotor-like rhythmic activity with similar characteristics can be induced in the neonatal mouse preparation. In the lumbar spinal cord in the neonatal mouse, it is likely that the rhythm-generating network is distributed throughout the lumbar region with a rostro-caudal gradient, a situation similar to that in the neonatal and fetal rat spinal cord. With this review we hope to highlight the dramatic changes that neuronal networks generating locomotor-like activity undergo during the prenatal development of the rat. Moreover, the distribution of the neuronal network generating the locomotor rhythm in the neonatal rat and mouse spinal cord is compared.

Ubiquity of motor networks in the spinal cord of vertebrates

In a recent paper, we found that it is possible to record motor activity in sacral segments in the in vitro neonatal rat spinal cord preparation. This motor activity recorded in segments that are not innervating hindlimbs is driven by the lumbar locomotor network. Indeed, compartimentalizations of the cord with Vaseline walls or section experiments, reveals that the sacral segments possess their own rhythmogenic capabilities but that in an intact spinal cord they are driven by the lumbar locomotor network. In this review, these recent findings are placed in the context of spinal motor network interactions. As previously suspected, the motor networks do not operate in isolation but interact with each other according to behavioural needs. These interactions provide some insight into the discrepancies observed in several studies dealing with the localization of the lumbar locomotor network in the neonatal rat spinal cord. In conclusion, the spinal cord of quadrupeds appears as an heterogeneous structure where it is possible to identify neuronal networks that are crucial for the genesis of locomotor-related activities.

Regulation by glycine, Mg2+ and polyamines of the N-methyl-D-aspartate-induced locomotion in the neonatal rat spinal cord in vitro

Excitatory amino acids are known to activate the spinal neural network that organize locomotor activity in various species. In this study, the role of various compounds which alter the functioning of the N-methyl-D-aspartate receptor (glycine, Mg2+ ions and spermine) was investigated during fictive locomotion, using an in vitro isolated spinal cord preparation from neonatal rats. Locomotor-like activity induced by excitatory amino acids was recorded both extra- and intracellularly. 7-chloro-kynurenic acid, an antagonist of the glycine site at the N-methyl-D-aspartate receptor, depressed the N-methyl-D-aspartate component of the synaptic inputs received by the motoneurons. Glycine at low concentrations had no effect on locomotor activity, while 7-chlorokynurenic acid increased the locomotor period and decreased the burst amplitude in a dose-dependent manner. Removal of Mg2+ ions from the saline facilitated the N-methyl-D-aspartate-mediated response, and triggered spontaneous bursting activity, abolished by 2-amino-5-phosphonovaleric acid, an antagonist of the N-methyl-D-aspartate receptor. The polyamine, spermine, did not change the locomotor parameters. On the contrary, arcaine, a putative antagonist of the polyamine site on the N-methyl-D-aspartate receptor, increased locomotor activity. The effects of arcaine were counteracted by spermine. These results suggest that glycine and spermine are present at saturating concentrations on the N-methyl-D-aspartate receptor during ongoing locomotion. Together with Mg2+ ions, these endogenous regulators contribute to control the level of activity of the N-methyl-D-aspartate receptor in the spinal cord of the neonatal rat.

Sacrocaudal afferents induce rhythmic efferent bursting in isolated spinal cords of neonatal rats

The ability of mammalian spinal cords to generate rhythmic motor behavior in nonlimb moving segments was examined in isolated spinal cords of neonatal rats. Stimulation of sacrocaudal afferents (SCA) induced alternating left-right bursts in lumbosacral efferents and in tail muscles. On each side of the tail, flexors, extensors, and abductors were coactive during each cycle of activity. This rhythm originated mainly in the sacrocaudal region because it persisted in sacrocaudal segments after surgical removal of the thoracolumbar cord. Sacrocaudal commissural pathways were sufficient to maintain the left-right alternation of lumbar efferent bursts, because their timing was unaltered after a complete thoracolumbar hemisection. The lumbar rhythm originated in part from sacrocaudal activity ascending in lateral and ventrolateral funiculi, because efferent bursts in rostral lumbar segments were nearly abolished on a particular side by lesions of the lateral quadrant of the cord at the L(4)-L(5) junction. Intracellular recordings from S(2)-S(3) motoneurons, obtained during the rhythm, revealed the presence of phasic oscillations of membrane potential superimposed on a tonic depolarization. Bursts of spikes occurred on the depolarizing phases of the oscillation. Between these bursts the membrane input conductance increased, and hyperpolarizing drive potentials were revealed. The inhibitory drive and the decreased input resistance coincided with contralateral efferent bursts, suggesting that crossed pathways controlled it. Our studies indicate that pattern generators are not restricted to limb-moving spinal segments and suggest that regional specializations of pattern-generating circuitry and their associated interneurons are responsible for the different motor patterns produced by the mammalian spinal cord.

Connecting brains to robots: an artificial body for studying the computational properties of neural tissues

We have created a hybrid neuro-robotic system that establishes two-way communication between the brain of a lamprey and a small mobile robot. The purpose of this system is to offer a new paradigm for investigating the behavioral, computational, and neurobiological mechanisms of sensory-motor learning in a unified context. The mobile robot acts as an artificial body that delivers sensory information to the neural tissue and receives command signals from it. The sensory information encodes the intensity of light generated by a fixed source. The closed-loop interaction between brain and robot generates autonomous behaviors whose features are strictly related to the structure and operation of the neural preparation. We provide a detailed description of the hybrid system, and we present experimental findings on its performance. In particular, we found (a) that the hybrid system generates stable behaviors, (b) that different preparations display different but systematic responses to the presentation of an optical stimulus, and (c) that alteration of the sensory input leads to short- and long-term adaptive changes in the robot responses. The comparison of the behaviors generated by the lamprey's brain stem with the behaviors generated by network models of the same neural system provides us with a new tool for investigating the computational properties of synaptic plasticity.

Tapping into spinal circuits to restore motor function

Motivated by the challenge of improving neuroprosthetic devices, the authors review current knowledge relating to harnessing the potential of spinal neural circuits, such as reflexes and pattern generators. If such spinal interneuronal circuits could be activated, they could provide the coordinated control of many muscles that is so complex to implement with a device that aims to address each participating muscle individually. The authors' goal is to identify candidate spinal circuits and areas of research that might open opportunities to effect control of human limbs through electrical activation of such circuits. David McCrea's discussion of the ways in which hindlimb reflexes in the cat modify motor activity may help in developing optimal strategies for functional neuromuscular stimulation (FNS), by using knowledge of how reflex actions can adapt to different conditions. Michael O'Donovan's discussion of the development of rhythmogenic networks in the chick embryo may provide clues to methods of generating rhythmic activity in the adult spinal cord. Serge Rossignol examines the spinal pattern generator for locomotion in cats, its trigger mechanisms, modulation and adaptation, and suggests how this knowledge can help guide therapeutic approaches in humans. Hugues Barbeau applies the work of Rossignol and others to locomotor training in human subjects who have suffered spinal cord injury (SCI) with incomplete motor function loss (IMFL). Michel Lemay and Warren Grill discuss some of the technical challenges that must be addressed by engineers to implement a neuroprosthesis using electrical stimulation of the spinal cord, particularly the control issues that would have to be resolved.

Coordination of limb movements: three types of intersegmental interneurons in the swimmeret system and their responses to changes in excitation

Coordination of limb movements: three types of intersegmental interneurons in the swimmeret system and their responses to changes in excitation. During forward locomotion, the movements of swimmerets on different segments of the crayfish abdomen are coordinated so that more posterior swimmerets lead their anterior neighbors by approximately 25%. This coordination is accomplished by mechanisms within the abdominal nerve cord. Here we describe three different types of intersegmental swimmeret interneurons that are necessary and sufficient to accomplish this coordination. These interneurons could be identified both by their structures within their home ganglion and by their physiological properties. These interneurons occur as bilateral pairs in each ganglion that innervates swimmerets, and their axons traverse the minuscule tract (MnT) of their home ganglion before leaving to project to neighboring ganglia. Two types, ASCE and ASCL, projected an axon anteriorly; the third type, DSC, projected posteriorly. Each type fires a burst of impulses starting at a different phase of the swimmeret cycle in its home ganglion. In active preparations, excitation of individual ASCE or DSC interneurons at different phases in the cycle affected the timing of the next cycle in the interneuron's target ganglion. The axons of these interneurons that projected between two ganglia ran close together, and their firing often could be recorded by the same electrode. Experiments in which either this tract or the rest of the intersegmental connectives was cut bilaterally showed that these interneurons were both necessary and sufficient for coordination of neighboring swimmerets. When the level of excitation of the swimmeret system was increased by bath application of carbachol, the period of the system's cycle shortened, but the characteristic phase difference within and between ganglia was preserved. Each of these interneurons responded to this increase in excitation by increasing the frequency of impulses within each burst, but the phases and relative durations of their bursts did not change, and their activity remained coordinated with the cycle in their home ganglion. The decrease in duration of each burst was matched to the increase in impulse frequency within the burst so that the mean numbers of impulses per burst did not change significantly despite a threefold change in period. These three types of interneurons appear to form a concatenated intersegmental coordinating circuit that imposes a particular intersegmental phase on the local pattern generating modules innervating each swimmeret. This circuit is asymmetric, and forces posterior segments to lead each cycle of output.

Intersegmental coordination in invertebrates and vertebrates

How does the CNS coordinate muscle contractions between different body segments during normal locomotion? Work on several preparations has shown that this coordination relies on excitability gradients and on differences between ascending and descending intersegmental coupling. Abstract models involving chains of coupled oscillators have defined properties of coordinating circuits that would permit them to establish a constant intersegmental phase in the face of changing periods. Analyses that combine computational and experimental strategies have led to new insights into the cellular organization of intersegmental coordinating circuits and the neural control of swimming in lamprey, tadpole, crayfish and leech.

Proprioceptive control of posture: a review of new concepts

The assumption that proprioceptive inputs from the lower legs are used to trigger balance and gait movements is questioned in this review (an outgrowth of discussions initiated during the Neural Control of Movement Satellite meeting held in Cozumel, Mexico, April 1997). Recent findings presented here suggest that trunk or hip inputs may be more important in triggering human balance corrections and that proprioceptive input from the lower legs mainly helps with the final shaping and intermuscular coordination of postural and gait movements. Three major questions were considered. First, what role, if any, do lower-leg proprioceptive inputs play in the triggering of normal balance corrections? If this role is negligible, which alternative proprioceptive inputs then trigger balance corrections? Second, what is the effect of proprioceptive loss on the triggering of postural and gait movements? Third, how does proprioceptive loss affect the output of central pattern generators in providing the final shaping of postural movements? The authors conclude that postural and gait movements are centrally organized at two levels. The first level involves the generation of the basic directional-specific response pattern based primarily on hip or trunk proprioceptive input secondarily on vestibular inputs. This pattern specifies the spatial characteristics of muscle activation, that is which muscles are primarily activated, as well as intermuscular timing, that is, the sequence in which muscles are activated. The second level is involved in the shaping of centrally set activation patterns on the basis of multisensorial afferent input (including proprioceptive input from all body segments and vestibular sensors) in order that movements can adapt to different task conditions. Copyright 1998 Elsevier Science B.V.