An inter-segmental network model and its use in elucidating gait-switches in the stick insect

Animal locomotion requires highly coordinated working of the segmental neuronal networks that control the limb movements. Experiments have shown that sensory signals originating from the extremities play a pivotal role in controlling locomotion patterns by acting on central networks. Based on the results from stick insect locomotion, we constructed an inter-segmental model comprising local networks for all three legs, i.e. for the pro-, meso- and meta-thorax, their inter-connections and the main sensory inputs modifying their activities. In the model, the local networks are uniform, and each of them consists of a central pattern generator (CPG) providing the rhythmic oscillation for the protractor-retractor motor systems, the corresponding motoneurons (MNs), and local inhibitory interneurons (IINs) between the CPGs and the MNs. Between segments, the CPGs are connected cyclically by both excitatory and inhibitory pathways that are modulated by the aforementioned sensory inputs. Simulations done with our network model showed that it was capable of reproducing basic patterns of locomotion such as those occurring during tri- and tetrapod gaits. The model further revealed a number of elementary neuronal processes (e.g. synaptic inhibition, or changing the synaptic drive at specific neurons) that in the simulations were necessary, and in their entirety sufficient, to bring about a transition from one type of gait to another. The main result of this simulation study is that exactly the same mechanism underlies the transition between the two types of gait irrespective of the direction of the change. Moreover, the model suggests that the majority of these processes can be attributed to direct sensory influences, and changes are required only in centrally controlled synaptic drives to the CPGs.     

Central pattern generating networks in insect locomotion

Central pattern generators (CPGs) are neural circuits that based on their connectivity can generate rhythmic and patterned output in the absence of rhythmic external inputs. This property makes CPGs crucial elements in the generation of many kinds of rhythmic motor behaviors in insects, such as flying, walking, swimming, or crawling. Arguably representing the most diverse group of animals, insects utilize at least one of these types of locomotion during one stage of their ontogenesis. Insects have been extensively used to study the neural basis of rhythmic motor behaviors, and particularly the structure and operation of CPGs involved in locomotion. Here, we review insect locomotion with regard to flying, walking, and crawling, and we discuss the contribution of central pattern generation to these three forms of locomotion. In each case, we compare and contrast the topology and structure of the CPGs, and we point out how these factors are involved in the generation of the respective motor pattern. We focus on the importance of sensory information for establishing a functional motor output and we indicate behavior‐specific adaptations. Furthermore, we report on the mechanisms underlying coordination between different body parts. Last but not least, by reviewing the state‐of‐the‐art knowledge concerning the role of CPGs in insect locomotion, we endeavor to create a common ground, upon which future research in the field of motor control in insects can build.     

Partly shared spinal cord networks for locomotion and scratching

Animals produce a variety of behaviors using a limited number of muscles and motor neurons. Rhythmic behaviors are often generated in basic form by networks of neurons within the central nervous system, or central pattern generators (CPGs). It is known from several invertebrates that different rhythmic behaviors involving the same muscles and motor neurons can be generated by a single CPG, multiple separate CPGs, or partly overlapping CPGs. Much less is known about how vertebrates generate multiple, rhythmic behaviors involving the same muscles. The spinal cord of limbed vertebrates contains CPGs for locomotion and multiple forms of scratching. We investigated the extent of sharing of CPGs for hind limb locomotion and for scratching. We used the spinal cord of adult red-eared turtles. Animals were immobilized to remove movement-related sensory feedback and were spinally transected to remove input from the brain. We took two approaches. First, we monitored individual spinal cord interneurons (i.e., neurons that are in between sensory neurons and motor neurons) during generation of each kind of rhythmic output of motor neurons (i.e., each motor pattern). Many spinal cord interneurons were rhythmically activated during the motor patterns for forward swimming and all three forms of scratching. Some of these scratch/swim interneurons had physiological and morphological properties consistent with their playing a role in the generation of motor patterns for all of these rhythmic behaviors. Other spinal cord interneurons, however, were rhythmically activated during scratching motor patterns but inhibited during swimming motor patterns. Thus, locomotion and scratching may be generated by partly shared spinal cord CPGs. Second, we delivered swim-evoking and scratch-evoking stimuli simultaneously and monitored the resulting motor patterns. Simultaneous stimulation could cause interactions of scratch inputs with subthreshold swim inputs to produce normal swimming, acceleration of the swimming rhythm, scratch-swim hybrid cycles, or complete cessation of the rhythm. The type of effect obtained depended on the level of swim-evoking stimulation. These effects suggest that swim-evoking and scratch-evoking inputs can interact strongly in the spinal cord to modify the rhythm and pattern of motor output. Collectively, the single-neuron recordings and the results of simultaneous stimulation suggest that important elements of the generation of rhythms and patterns are shared between locomotion and scratching in limbed vertebrates.     

Predictability of visual perturbation during locomotion: implications for corrective efference copy signaling

In guiding adaptive behavior, efference copy signals or corollary discharge are traditionally considered to serve as predictors of self-generated sensory inputs and by interfering with their central processing are able to counter unwanted consequences of an animal??s own actions. Here, in a speculative reflection on this issue, we consider a different functional role for such intrinsic predictive signaling, namely in stabilizing gaze during locomotion where resultant changes in head orientation in space require online compensatory eye movements in order to prevent retinal image slip. The direct activation of extraocular motoneurons by locomotor-related efference copies offers a prospective substrate for assisting self-motion derived sensory feedback, rather than being subtracted from the sensory signal to eliminate unwanted reafferent information. However, implementing such a feed-forward mechanism would be critically dependent on an appropriate phase coupling between rhythmic propulsive movement and resultant head/visual image displacement. We used video analyzes of actual locomotor behavior and basic theoretical modeling to evaluate head motion during stable locomotion in animals as diverse as Xenopus laevis tadpoles, teleost fish and horses in order to assess the potential suitability of spinal efference copies to the stabilization of gaze during locomotion. In all three species, and therefore regardless of aquatic or terrestrial environment, the head displacements that accompanied locomotor action displayed a strong correlative spatio-temporal relationship in correspondence with a potential predictive value for compensatory eye adjustments. Although spinal central pattern generator-derived efference copies offer appropriately timed commands for extraocular motor control during self-generated motion, it is likely that precise image stabilization requires the additional contributions of sensory feedback signals. Nonetheless, the predictability of the visual consequences of stereotyped locomotion renders intrinsic efference copy signaling an appealing mechanism for offsetting these disturbances, thus questioning the exclusive role traditionally ascribed to sensory-motor transformations in stabilizing gaze during vertebrate locomotion.     

Spinal and sensory neuromodulation of spinal neuronal networks in humans

The present experiments were designed to gain additionally insight into how the spinal networks process direct spinal stimulation and peripheral sensory inputs to control posture and locomotor movements. We have developed a plantar pressure stimulation system that can deliver naturalistic postural and gait-related patterns of pressure to the soles of the feet to simulate standing and walking, thereby activating and/or modulating the automated spinal circuitry responsible for standing and locomotion. In the present study we compare the patterns of activation among selected motor pools and the kinematic consequences of these activation patterns in response to patterned heel-to-toe mechanical stimulation of the soles of the feet, and/or transcutaneous electrical spinal stimulation, for postural and locomotion regulation. The studies were performed in healthy individuals (n = 12) as well as in subjects (n = 2) with motor complete spinal cord injury. We found that plantar pressure stimulation and/or spinal stimulation can effectively facilitate locomotor output in the subjects placed with their legs in gravity neutral position. We have shown synergistic effects of combining sensory and spinal cord stimulation, suggesting that the two networks are different, but complementary. Also we provide evidence that plantar stimulation could serve as a novel neuro-rehabilitation tool alone or as part of a multi-modal approach to restoring motor function after complete paralysis due to SCI.     

Spinal pattern generation and sensory gating mechanisms

Sensory gating mechanisms are deployed during vertebrate locomotion to ensure that adaptive and appropriate motor responses to afferent input occur during all phases of the movement cycle. Recent animal studies on the integration of cutaneous information have investigated the roles of interneurones in sensory gating. Premotor interneurones, rhythmically active during locomotion, as well as 'sensory' interneurones appear to be intimately involved in sensory gating, receiving synaptic inputs from the spinal rhythm generator to gate the flow of sensory information in the spinal cord.     

Comparison between multiple behavioral effects of peripheral ethanol administration in rats: sedation, ataxia, and bradykinesia

Although low doses of systemic ethanol stimulate locomotion in mice, in rats the typical response to peripheral ethanol administration is a dose-dependent suppression of motor activity. In the present study, male rats received acute doses of ethanol IP (0.0, 0.25, 0.5, 1.0 or 2.0 g/kg) and were tested on several behavioral tasks related to the motor suppressive or sedative effects of the drug. This research design allowed for comparisons between the effects of ethanol on different behavioral tasks in order to determine which tasks were most sensitive to the drug (i.e., which tasks would yield deficits that appear at lower doses). In the first two experiments, rats were evaluated on a sedation rating scale, and ataxia/motor incoordination was assessed using the rotarod apparatus. Administration of 2.0 g/kg ethanol produced sedation as measured by the sedation scale, and also impaired performance on the rotarod. In a third experiment, ethanol reduced locomotion in the stabilimeter at several doses and times after IP injection, with 0.25 g/kg being the lowest dose that produced a significant decrease in locomotion. Finally, experiment four studied the effects of ethanol on operant lever pressing reinforced on a fixed ratio 5 (FR5) schedule for food reinforcement. Data showed suppressive effects on lever pressing at doses of 1.0, and 2.0 g/kg ethanol. Analysis of the interresponse time distribution showed that ethanol produced a modest slowing of operant responding, as well as fragmentation of the temporal pattern of responding and increases in pausing. Taken together, these results indicate that rats can demonstrate reduced locomotion and slowing of operant responding at doses lower than those that result in sedation or ataxia as measured by the rotarod. The detection of subtle changes in different motor test across a broad range of ethanol doses is important for understanding ethanol effects in other cognitive, motivational or sensory processes.     

Proprioception,the regulator of motor function

In animals, proper locomotion is crucial to find mates and foods and avoid predators or dangers. Multiple sensory systems detect external and internal cues and integrate them to modulate motor outputs. Proprioception is the internal sense of body position, and proprioceptive control of locomotion is essential to generate and maintain precise patterns of movement or gaits. This proprioceptive feedback system is conserved in many animal species and is mediated by stretch-sensitive receptors called proprioceptors. Recent studies have identified multiple proprioceptive neurons and proprioceptors and their roles in the locomotion of various model organisms. In this review we describe molecular and neuronal mechanisms underlying proprioceptive feedback systems in C. elegans, Drosophila, and mice.     

The neural circuits and synaptic mechanisms underlying motor initiation in C. elegans

C. elegans is widely used to dissect how neural circuits and genes generate behavior. During locomotion, worms initiate backward movement to change locomotion direction spontaneously or in response to sensory cues; however, the underlying neural circuits are not well defined. We applied a multidisciplinary approach to map neural circuits in freely behaving worms by integrating functional imaging, optogenetic interrogation, genetic manipulation, laser ablation, and electrophysiology. We found that a disinhibitory circuit and a stimulatory circuit together promote initiation of backward movement and that circuitry dynamics is differentially regulated by sensory cues. Both circuits require glutamatergic transmission but depend on distinct glutamate receptors. This dual mode of motor initiation control is found in mammals, suggesting that distantly related organisms with anatomically distinct nervous systems may adopt similar strategies for motor control. Additionally, our studies illustrate how a multidisciplinary approach facilitates dissection of circuit and synaptic mechanisms underlying behavior in a genetic model organism.     

Identified target motor neuron regulates neurite outgrowth and synapse formation of aplysia sensory neurons in vitro

To determine the influence that an appropriate target cell has on the axonal structure of a presynaptic neuron in vivo, we examined the morphologies of individual Aplysia sensory neurons in dissociated cell culture in the presence or absence of identified target motor neurons. We find that an appropriate target, the motor cell L7, regulates the morphological differentiation of the presynaptic sensory neurons in two ways: the target induces the axons of the sensory neurons to develop a more elaborate structure and to form active zones, and the target guides the outgrowth of the sensory neurons. The influence of the appropriate target, L7, on the morphological differentiation of sensory neurons appears to be related to the formation of chemical synaptic connections between the sensory neurons and L7, since sensory neurons co-cultured with an inappropriate target motor neuron do not exhibit a comparable elaboration of their axonal processes.     

Experiments and models of sensorimotor interactions during locomotion

During locomotion sensory information from cutaneous and muscle receptors is continuously integrated with the locomotor central pattern generator (CPG) to generate an appropriate motor output to meet the demands of the environment. Sensory signals from peripheral receptors can strongly impact the timing and amplitude of locomotor activity. This sensory information is gated centrally depending on the state of the system (i.e., rest vs. locomotion) but is also modulated according to the phase of a given task. Consequently, if one is to devise biologically relevant walking models it is imperative that these sensorimotor interactions at the spinal level be incorporated into the control system.     

Caterpillar crawling over irregular terrain: anticipation and local sensing

Animal locomotion is produced by co-coordinated patterns of motor activity that are generally organized by central pattern generators and modified by sensory feedback. Animals with remote sensing can anticipate obstacles and make adjustments in their gait to accommodate them. It is largely unknown how animals that rely on touch might use such information to adjust their gait. One possibility is immediate (reflexive) change in motor activity. Elongated animals, however, might modulate movements by passing information from anterior to posterior segments. Using the caterpillar Manduca sexta we examined the movements of the most anterior abdominal prolegs as they approached an obstacle. The first pair of prolegs anticipated the obstacle by lifting more quickly in the earliest part of the swing phase: the caterpillar had information about the obstacle at proleg lift-off. Sometimes the prolegs corrected their trajectory mid-step. Removal of sensory hairs on the stepping leg did not affect the early anticipatory movements, but did change the distance at which the mid-step corrections occurred. We conclude that anterior sensory information can be passed backwards and used to modulate an ongoing crawl. The local sensory hairs on each body segment can then fine-tune movements of the prolegs as they approach an obstacle.     

Sensory pathways and their modulation in the control of locomotion

Recent experiments have extended our understanding of how sensory information in premotor networks controlling motor output is processed during locomotion, and at what level the efficacy of specific sensory—motor pathways is determined. Phasic presynaptic inhibition of sensory transmission combined with postsynaptic alterations of excitatory and inhibitory synaptic transmission from interneurons of the premotor networks contribute to the modulation of reflex pathways and to the generation of reflex reversal. These mechanisms play an important role in adapting the operation of central networks to external demands and thus help optimize sensory—motor integration.     

Establishment of vagal sensorimotor circuits during fetal development in rats

The differentiation of vagal motor neurons and their emerging central relationship with vagal sensory afferents was examined in fetal rats. To identify peripherally projecting sensory and motor neurons, 1,1′-dioctadecyl 3,3,3′,3′-tetramethylindocarbocyanine perchloarate (DiI) was inserted into the proximal gut or cervical vagus nerve in fixed preparations. At embryonic day (E) 12, labeled vagal sensory neurons are present in the nodose ganglia and a few sensory axons project into the dorsolateral medulla. Central sensory processes become increasingly prevalent between E13 and E14 but remain restricted to the solitary tract. Vagal motor neurons are first labeled at E13, clustered within a region corresponding to the nucleus ambiguus (NA). Additional motor neurons appear to be migrating toward the NA from the germinal zone of the fourth ventricle. Motor neurons in the dorsal motor nucleus of the vagus (DMV) first project to the gut at E14 and have processes that remain in physical contact with the ventricular zone through E16. Sensory axons emerge from the solitary tract at E15 and project medially through the region of the nucleus of the solitary tract (NST) to end in the ventricular zone. A possible substrate for direct vagovagal, sensorimotor interaction appears at E16, when vagal sensory fibers arborize within the DMV and DMV dendrites extend into the NST. By E18, the vagal nuclei appear remarkably mature. These data suggest specific and discrete targeting of vagal sensory afferents and motor neuron dendrites in fetal rats and define an orderly sequence of developmental events that precedes the establishment of vagal sensorimotor circuits. © 1993 John Wiley & Sons, Inc.     

Elimination of sensory inputs induces growth and synaptic changes in crayfish motor axons

Damage to motor neurons induces regeneration processes including axonal growth and change of synaptic properties. Sensory axons that run along the motor axons are also damaged, but their possible role in the motor neuron''s regeneration is generally ignored. Here, the effect of eliminating some sensory inputs from intact motor axons on the motor axon''s properties was studied. Micro-dissecting one of the segmental, bilateral, sensory stretch receptor pairs of the crayfish abdomen induced the deep extensor abdominal motor axons to grow and changed their synaptic properties. The results demonstrate directly, probably for the first time, that change in sensory neuron activity can induce motor axons to grow, form new synapses, and change their synaptic properties.     

Role of Sensory Experience in Functional Development of Drosophila Motor Circuits

Neuronal circuits are formed according to a genetically predetermined program and then reconstructed in an experience-dependent manner. While the existence of experience-dependent plasticity has been demonstrated for the visual and other sensory systems, it remains unknown whether this is also the case for motor systems. Here we examined the effects of eliminating sensory inputs on the development of peristaltic movements in Drosophila embryos and larvae. The peristalsis is initially slow and uncoordinated, but gradually develops into a mature pattern during late embryonic stages. We tested whether inhibiting the transmission of specific sensory neurons during this period would have lasting effects on the properties of the sensorimotor circuits. We applied Shibire-mediated inhibition for six hours during embryonic development (15–21 h after egg laying [AEL]) and studied its effects on peristalsis in the mature second- and third-instar larvae. We found that inhibition of chordotonal organs, but not multidendritic neurons, led to a lasting decrease in the speed of larval locomotion. To narrow down the sensitive period, we applied shorter inhibition at various embryonic and larval stages and found that two-hour inhibition during 16–20 h AEL, but not at earlier or later stages, was sufficient to cause the effect. These results suggest that neural activity mediated by specific sensory neurons is involved in the maturation of sensorimotor circuits in Drosophila and that there is a critical period for this plastic change. Consistent with a role of chordotonal neurons in sensory feedback, these neurons were activated during larval peristalsis and acute inhibition of their activity decreased the speed of larval locomotion.     

Npn-1 contributes to axon-axon interactions that differentially control sensory and motor innervation of the limb

The initiation, execution, and completion of complex locomotor behaviors are depending on precisely integrated neural circuitries consisting of motor pathways that activate muscles in the extremities and sensory afferents that deliver feedback to motoneurons. These projections form in tight temporal and spatial vicinities during development, yet the molecular mechanisms and cues coordinating these processes are not well understood. Using cell-type specific ablation of the axon guidance receptor Neuropilin-1 (Npn-1) in spinal motoneurons or in sensory neurons in the dorsal root ganglia (DRG), we have explored the contribution of this signaling pathway to correct innervation of the limb. We show that Npn-1 controls the fasciculation of both projections and mediates inter-axonal communication. Removal of Npn-1 from sensory neurons results in defasciculation of sensory axons and, surprisingly, also of motor axons. In addition, the tight coupling between these two heterotypic axonal populations is lifted with sensory fibers now leading the spinal nerve projection. These findings are corroborated by partial genetic elimination of sensory neurons, which causes defasciculation of motor projections to the limb. Deletion of Npn-1 from motoneurons leads to severe defasciculation of motor axons in the distal limb and dorsal-ventral pathfinding errors, while outgrowth and fasciculation of sensory trajectories into the limb remain unaffected. Genetic elimination of motoneurons, however, revealed that sensory axons need only minimal scaffolding by motor axons to establish their projections in the distal limb. Thus, motor and sensory axons are mutually dependent on each other for the generation of their trajectories and interact in part through Npn-1-mediated fasciculation before and within the plexus region of the limbs.     

Analysis of the mechanisms of interaction between generators of cyclic motor reactions and inputs from suprasegmental systems

The simulation mathematical model of neuronal generator systems was used for analyzing the interaction between inputs from descending (afferent) systems and generators of scratching (locomotion). The data obtained indicate that in cases when generators of cyclic motor reactions influence the effectiveness of synaptic transmission from the fibers of descending (or afferent) systems, the differences in the main characteristics of signals that are produced by these generators themselves and those that come to them are emphasized. These data allow us to conclude that the system supplying interaction between inputs from suprasegmental (afferent) fiber systems and generators of locomotion of scratching can be interpreted as an adaptive filter which processes spatial and temporal information coming to the spinal cord via different suprasegmental or primary afferent inputs and allows generators of cyclic motor reactions to correct their functioning in accordance with changing external conditions.Neirofiziologiya/Neurophysiology, Vol. 25, No. 3, pp. 211–215, May–June, 1993.     

Sensorimotor integration: locating locomotion in neural circuits

Neural components of the circuits that transform sensory cues into changes in motor activities are largely unknown. Several recent studies have now functionally mapped the sensorimotor circuits responsible for locomotion behaviors under defined environmental conditions in the nematode Caenorhabditis elegans.     

Axonal degeneration within the tympanal nerve of Schistocerca gregaria

This study describes time course and ultrastructural changes during axonal degeneration of different neurones within the tympanal nerve of the locust Schistocerca gregaria. The tympanal nerve innervates the tergit and pleurit of the first abdominal segment and contains the axons of both sensory and motor neurones. The majority of axons (approx. 97%) belong to several types of sensory neurones: mechano- and chemosensitive hair sensilla, multipolar neurones, campaniform sensilla and sensory cells of a scolopidial organ, the auditory organ. Axons of campaniform sensilla, of auditory sensory cells and of motor neurones are wrapped by glial cell processes. In contrast, the very small and numerous axons (diameter <1 microm) of multipolar neurones and hair sensilla are not separated individually by glia sheets. Distal parts of sensory and motor axons show different reactions to axotomy: 1 week after separation from their somata, distal parts of motor axons are invaded by glial cell processes. This results in fascicles of small axon bundles. In contrast, distal parts of most sensory axons degenerate rapidly after being lesioned. The time to onset of degeneration depends on distance from the lesion site and on the type of sensory neurone. In axons of auditory sensory neurones, ultrastructural signs of degeneration can be found as soon as 2 days after lesion. After complete lysis of distal parts of axons, glial cell processes invade the space formerly occupied by sensory axons. The rapid degeneration of distal auditory axon parts allows it to be excluded that they provide a structure that leads regenerating axons to their targets. Proximal parts of severed axons do not degenerate.