Intersegmental transfer of sensory signals in the stick insect leg muscle control system

Intersegmental coordination during locomotion in legged animals arises from mechanical couplings and the exchange of neuronal information between legs. Here, the information flow from a single leg sense organ of the stick insect Cuniculina impigra onto motoneurons and interneurons of other legs was investigated. The femoral chordotonal organ (fCO) of the right middle leg, which measures posture and movement of the femur-tibia joint, was stimulated, and the responses of the tibial motoneuron pools of the other legs were recorded. In resting animals, fCO signals did not affect motoneuronal activity in neighboring legs. When the locomotor system was activated and antagonistic motoneurons were bursting in alternation, fCO stimuli facilitated transitions from flexor to extensor activity and vice versa in the contralateral leg. Following pharmacological treatment with picrotoxin, a blocker of GABA-ergic inhibition, the tibial motoneurons of all legs showed specific responses to signals from the middle leg fCO. For the contralateral middle leg we show that fCO signals encoding velocity and position of the tibia were processed by those identified local premotor nonspiking interneurons known to contribute to posture and movement control during standing and voluntary leg movements. Interneurons received both excitatory and inhibitory inputs, so that the response of some interneurons supported the motoneuronal output, while others opposed it. Our results demonstrate that sensory information from the fCO specifically affects the motoneuronal activity of other legs and that the layer of premotor nonspiking interneurons is a site of interaction between local proprioceptive sensory signals and proprioceptive signals from other legs.     

Identified nonspiking interneurons in leg reflexes and during walking in the stick insect

In the stick insect Carausius morosus identified nonspiking interneurons (type E4) were investigated in the mesothoracic ganglion during intraand intersegmental reflexes and during searching and walking.In the standing and in the actively moving animal interneurons of type E4 drive the excitatory extensor tibiae motoneurons, up to four excitatory protractor coxae motoneurons, and the common inhibitor 1 motoneuron (Figs. 1–4).In the standing animal a depolarization of this type of interneuron is induced by tactile stimuli to the tarsi of the ipsilateral front, middle and hind legs (Fig. 5). This response precedes and accompanies the observed activation of the affected middle leg motoneurons. The same is true when compensatory leg placement reflexes are elicited by tactile stimuli given to the tarsi of the legs (Fig. 6).During forward walking the membrane potential of interneurons of type E4 is strongly modulated in the step-cycle (Figs.8–10). The peak depolarization occurs at the transition from stance to swing. The oscillations in membrane potential are correlated with the activity profile of the extensor motoneurons and the common inhibitor 1 (Fig. 9).The described properties of interneuron type E4 in the actively behaving animal show that these interneurons are involved in the organization and coordination of the motor output of the proximal leg joints during reflex movements and during walking.Abbreviations CLP reflex, compensatory leg placement reflex - CI1 common inhibitor I motoneuron - fCO femoral chordotonal organ - FETi fast extensor tibiae motoneuron - FT femur-tibia - SETi slow extensor tibiae motoneuron     

Intracellular recordings from nonspiking interneurons in a semiintact, tethered walking insect.

Nonspiking interneurons were investigated in a tethered, walking insect, Carausius morosus, that was able to freely perform walking movements. Experiments were carried out with animals walking on a lightweight, double-wheel treadmill. Although the animal was opened dorsally, the walking system was left intact. Intracellular recordings were obtained from the dorsal posterior neuropil of the mesothoracic ganglion. Nonspiking interneurons, in which modulations of the membrane potential were correlated with the walking rhythm, were described physiologically and stained with Lucifer Yellow. Interneurons are demonstrated in which membrane potential oscillations mirror the leg position or show correlation with the motoneuronal activity of the protractor and retractor coxae muscles during walking. Other interneurons showed distinct hyperpolarizations at certain important trigger points in the step cycle, for example, at the extreme posterior position. Through electrical stimulation of single, nonspiking interneurons during walking, the motoneuronal activity in two antagonistic muscles--protractor and retractor coxae--could be reversed and even the movement of the ipsilateral leg could be influenced. The nonspiking interneurons described appear to be important premotor elements involved in walking. They receive, integrate, and process information from different leg proprioceptors and drive groups of leg motoneurons during walking.     

Intracellular recordings from nonspiking interneurons in a semiintact,tethered walking insect

Nonspiking interneurons were investigated in a tethered, walking insect, Carausius morosus, that was able to freely perform walking movements. Experiments were carried out with animals walking on a lightweight, double-wheel treadmill. Although the animal was opened dorsally, the walking system was left intact. Intracellular recordings were obtained from the dorsal posterior neuropil of the mesothoracic ganglion. Nonspiking inter-neurons, in which modulations of the membrane potential were correlated with the walking rhythm, were described physiologically and stained with Lucifer Yellow. Interneurons are demonstrated in which membrane potential oscillations mirror the leg position or show correlation with the motoneuronal activity of the protractor and retractor coxae muscles during walking. Other interneurons showed distinct hyperpolarizations at certain important trigger points in the step cycle, for example, at the extreme posterior position. Through electrical stimulation of single, nonspiking interneurons during walking, the motoneuronal activity in two antagonistic muscles—protractor and retractor coxae—could be reversed and even the movement of the ipsilateral leg could be influenced. The nonspiking interneurons described appear to be important premotor elements involved in walking. They receive, integrate, and process information from different leg proprioceptors and drive groups of leg motoneurons during walking.     

Identification of Inhibitory Premotor Interneurons Activated at a Late Phase in a Motor Cycle during Drosophila Larval Locomotion

Rhythmic motor patterns underlying many types of locomotion are thought to be produced by central pattern generators (CPGs). Our knowledge of how CPG networks generate motor patterns in complex nervous systems remains incomplete, despite decades of work in a variety of model organisms. Substrate borne locomotion in Drosophila larvae is driven by waves of muscular contraction that propagate through multiple body segments. We use the motor circuitry underlying crawling in larval Drosophila as a model to try to understand how segmentally coordinated rhythmic motor patterns are generated. Whereas muscles, motoneurons and sensory neurons have been well investigated in this system, far less is known about the identities and function of interneurons. Our recent study identified a class of glutamatergic premotor interneurons, PMSIs (period-positive median segmental interneurons), that regulate the speed of locomotion. Here, we report on the identification of a distinct class of glutamatergic premotor interneurons called Glutamatergic Ventro-Lateral Interneurons (GVLIs). We used calcium imaging to search for interneurons that show rhythmic activity and identified GVLIs as interneurons showing wave-like activity during peristalsis. Paired GVLIs were present in each abdominal segment A1-A7 and locally extended an axon towards a dorsal neuropile region, where they formed GRASP-positive putative synaptic contacts with motoneurons. The interneurons expressed vesicular glutamate transporter (vGluT) and thus likely secrete glutamate, a neurotransmitter known to inhibit motoneurons. These anatomical results suggest that GVLIs are premotor interneurons that locally inhibit motoneurons in the same segment. Consistent with this, optogenetic activation of GVLIs with the red-shifted channelrhodopsin, CsChrimson ceased ongoing peristalsis in crawling larvae. Simultaneous calcium imaging of the activity of GVLIs and motoneurons showed that GVLIs’ wave-like activity lagged behind that of motoneurons by several segments. Thus, GVLIs are activated when the front of a forward motor wave reaches the second or third anterior segment. We propose that GVLIs are part of the feedback inhibition system that terminates motor activity once the front of the motor wave proceeds to anterior segments.     

GABAergic and glutamatergic inhibition of nonspiking local interneurons in the terminal abdominal ganglion of the crayfish

Nonspiking local interneurons in the terminal abdominal ganglion of the crayfish Procambarus clarkii receive inhibitory inputs from mainly glutamatergic spiking local interneurons and GABAergic nonspiking interneurons. In this study, the inhibitory responses of nonspiking interneurons to local application of glutamate and GABA into the neuropil were compared. Glutamate and GABA injection mediated the hyperpolarization of the nonspiking interneurons with an increase in membrane conductance. The glutamate-mediated membrane hyperpolarization was reversed by injection of 1 or 2 nA hyperpolarizing current. By contrast, more than 3 nA hyperpolarizing current was frequently necessary to reverse the GABA-mediated hyperpolarization. Bath application of a chloride channel blocker, 50 microM picrotoxin (PTX), reduced the glutamate-mediated hyperpolarization, but had no effect on the GABA-mediated hyperpolarization. The GABA-mediated hyperpolarization was not consistently affected by bath application of low chloride solution. These results suggest that the glutamate-mediated inhibition was related to the gating of a Cl(-) conductance, while the GABA-mediated inhibition was not. Electrical stimulation of sensory afferents innervating the exopodite elicited ipsps in uropod opener motor neurons. These sensory-evoked ipsps were also PTX-insensitive, suggesting GABAergic nonspiking interneurons could be the predominant premotor elements in organizing the uropod motor control system.     

Synaptic interactions between nonspiking local interneurones in the terminal abdominal ganglion of the crayfish

Nonspiking local interneurones are the important premotor elements in arthropod motor control systems. We have analyzed the synaptic interactions between nonspiking interneurones in the crayfish terminal (6th) abdominal ganglion using simultaneous intracellular recordings. Only 15% of nonspiking interneurones formed bi-directional excitatory connections. In 77% of connections, however, the nonspiking interneurones showed a one-way inhibitory interaction. In these cases, the presynaptic nonspiking interneurones received excitatory synaptic inputs from the sensory afferents innervating hairs on the surface of the uropods and the postsynaptic nonspiking interneurones received inhibitory synaptic inputs that were partly mediated by the inputs to the presynaptic nonspiking interneurones. The membrane hyperpolarization of the postsynaptic nonspiking interneurones mediated by the presynaptic nonspiking interneurones was reduced in amplitude when the hyperpolarizing current was injected into the postsynaptic interneurones, or when the external bathing solution was replaced with one containing low calcium and high magnesium concentrations. The role of these interactions in the circuits controlling the movements of the terminal appendages is discussed.Abbreviations AL antero-lateral - epsp excitatory postsynaptic potential - ipsp inhibitory postsynaptic potential - PL postero-lateral     

Premotor interneurons in generation of adaptive leg reflexes and voluntary movements in stick insects

We investigated the role of local nonspiking interneurons involved in motor control of legs in the stick insect, Carausius morosus. In a preparation that allowed the animals to perform active leg movements such as adaptive tactile reflexes, proprioceptive reflexes, and walking, we gathered the following results. Almost all tested nonspiking interneurons that provide synaptic drive onto motoneurons of the proximal leg muscles contribute to all of the motor programs underlying tactile reflexes and voluntary leg movements such as walking, searching, and rocking. Most of them are also involved in the generation of proprioceptive reflexes. All motor programs for coactivation, avoidance reflexes, resistance reflexes, and voluntary leg movements result from parallel pathways including nonspiking interneurons that support and others that oppose the motoneuronal activity. The contribution of a single interneuron to the different motor programs is specific: it can be supporting for one motor program but opposing for the other. Even for the same motor program, for example, coactivation, the contribution of an individual interneuron can depend on the stimulus site from where the response is elicited. Our results support the idea that the different motor patterns for adaptive tactile reflexes, resistance reflexes, and voluntary leg movements emerge from a multifunctional neuronal circuit that is reorganized corresponding to the motor behavior performed. The actual motor pattern is then shaped by distributed information processing in parallel supporting and opposing pathways. © 1996 John Wiley & Sons, Inc.     

Convergence of load and movement information onto leg motoneurons in insects

The interaction of two feedback loops was investigated: one regulating cuticular stress in the stick insect's leg and the other controlling leg posture. Exclusive stimulation of either of the two relevant sense organs, the load-sensitive trochantero-femoral campaniform sensilla (CS) or the position-/movement-sensitive ventral coxal hairplate (cxHPv), elicits resistance reflex responses in the retractor and the protractor coxae motoneuron pools. Concurrent application of both stimulus modalities reveals that the strength of the postural feedback response is dependent on sign and amplitude of the load feedback response and vice versa. This superposition of the two reflex responses appears to be non-linear. The results indicate that the CS information is underlying a force control function in this six-legged animal. It is hypothesized that the force control of each single leg could help to optimize the force distribution of the six-legged system, even - due to the mechanical coupling - without explicit neuronal pathways. On the level of the single leg control it was studied whether the different information provided by the two feedback transducers converge on the level of retractor coxae motoneurons or whether this information is fully preprocessed at the level of premotor interneurons. It is shown here that the hairplate afferents make direct, excitatory connections with the retractor motoneurons. Studies of the motoneurons' membrane conductances during exclusive CS stimulation reveal that both, excitatory as well as inhibitory synaptic drive is delivered onto the retractor motoneurons. Thus, the motoneuronal membrane is shown to be an important stage for the sensor fusion of the two modalities.     

An imbalancing act: gap junctions reduce the backward motor circuit activity to bias C. elegans for forward locomotion

A neural network can sustain and switch between different activity patterns to execute multiple behaviors. By monitoring the decision making for directional locomotion through motor circuit calcium imaging in?behaving Caenorhabditis elegans (C.?elegans), we reveal that C.?elegans determines the directionality of movements by establishing an imbalanced output between the forward and backward motor circuits and that it alters directions by switching between these imbalanced states. We further demonstrate that premotor interneurons modulate endogenous motoneuron activity to establish the output imbalance. Specifically, the UNC-7 and UNC-9 innexin-dependent premotor interneuron-motoneuron coupling prevents a balanced output state that leads to movements without directionality. Moreover, they act as shunts to decrease the backward-circuit activity, establishing a persistent bias for the high forward-circuit output state that results in the inherent preference of C.?elegans for forward locomotion. This study demonstrates that imbalanced motoneuron activity underlies directional movement and establishes gap junctions as critical modulators of the properties and outputs of neural circuits.     

Physiological changes of premotor nonspiking interneurons in the central compensation of eyestalk posture following unilateral sensory ablation in crayfish

We investigated how the physiological characteristics and synaptic activities of nonspiking giant interneurons (NGIs), which integrate sensory inputs in the brain and send synaptic outputs to oculomotor neurons innervating eyestalk muscles, changed after unilateral ablation of the statocyst in order to clarify neuronal mechanisms underlying the central compensation process in crayfish. The input resistance and membrane time constant in recovered animals that restored the original symmetrical eyestalk posture 2 weeks after operation were significantly greater than those immediately after operation on the operated side whereas in non-recovered animals only the membrane time constant showed a significant increase. On the intact side, both recovered and non-recovered animals showed no difference. The frequency of synaptic activity showed a complex pattern of change on both sides depending on the polarity of the synaptic potential. The synaptic activity returned to the bilaterally symmetrical level in recovered animals while bilateral asymmetry remained in non-recovered ones. These results suggest that the central compensation of eyestalk posture following unilateral impairment of the statocyst is subserved by not only changes in the physiological characteristics of the NGI membrane but also the activity of neuronal circuits presynaptic to NGIs.     

Simulation system of spinal cord motor nuclei and associated nerves and muscles,in a Web-based architecture

A Web-based simulation system of the spinal cord circuitry responsible for muscle control is described. The simulator employs two-compartment motoneuron models for S, FR and FF types, with synaptic inputs acting through conductance variations. Four motoneuron pools with their associated interneurons are represented in the simulator, with the possibility of inclusion of more than 2,000 neurons and 2,000,000 synapses. Each motoneuron action potential is followed, after a conduction delay, by a motor unit potential and a motor unit twitch. The sums of all motor unit potentials and twitches result in the electromyogram (EMG), and the muscle force, respectively. Inputs to the motoneuron pool come from populations of interneurons (Ia reciprocal inhibitory interneurons, Ib interneurons, and Renshaw cells) and from stochastic point processes associated with descending tracts. To simulate human electrophysiological experiments, the simulator incorporates external nerve stimulation with orthodromic and antidromic propagation. This provides the mechanisms for reflex generation and activation of spinal neuronal circuits that modulate the activity of another motoneuron pool (e.g., by reciprocal inhibition). The generation of the H-reflex by the Ia-motoneuron pool system and its modulation by spinal cord interneurons is included in the simulation system. Studies with the simulator may include the statistics of individual motoneuron or interneuron spike trains or the collective effect of a motor nucleus on the dynamics of muscle force control. Properties associated with motor-unit recruitment, motor-unit synchronization, recurrent inhibition and reciprocal inhibition may be investigated.     

Physiological changes in central neuronal pathways contributing to the generation of a reflex reversal

1. In the stick insect Carausius morosus the properties of the neuronal network governing the femur-tibia joint depend on the behavioral state of the animal. In the inactive animal flexion of the femur-tibia joint results in the generation of a resistance reflex, while in the active animal the same stimulus induces the so-called active reaction, the first part of which is a reflex reversal.
2. Recordings from motoneurons innervating the extensor tibiae muscle indicated that their time course of activity during the active reaction is due to inputs from intercalated pathways. We therefore investigated the role of identified nonspiking interneurons that transmit sensory information from the chordotonal organ onto the extensor motoneurons in the inactive animal. We can show that (i) the nonspiking interneurons received altered inputs whereas (ii) they provided qualitatively the same synaptic drive onto leg motoneurons.
3. From our results it is clear that (i) neuronal pathways contributing to the generation of the resistance reflex are also involved in the generation of the reflex reversal in the same control loop, (ii) thereby adopting the same principle of information processing (parliamentary principle), because both, supporting and opposing pathways contribute to the generation of the motor output.

     

Developmental attenuation of Manduca pre-ecdysis behavior involves neural changes upstream of motoneurons and relay interneurons

Each molt in the hawkmoth, Manduca sexta, culminates in the shedding of the old cuticle at ecdysis. Prior to each larval ecdysis, the old cuticle is loosened by pre-ecdysis behavior, which includes rhythmic, synchronous compressions in all abdominal segments. Prior to ecdysis to the pupal stage, pre-ecdysis behavior and its underlying motor pattern are markedly attenuated. A single pair of interneurons located in the terminal abdominal ganglion, the IN-402s, drives compression motoneuron activity during the pre-ecdysis motor pattern via monosynaptic excitatory connections. The present study tested the hypotheses that (1) changes in intrinsic properties (resting membrane potential, spike threshold, input resistance and excitability) of compression motoneurons, or (2) changes in the strength of synaptic connections from IN-402s to compression motoneurons, underlie the developmental attenuation of the pre-ecdysis motor pattern. Membrane potential was slightly more hyperpolarized in prepupal as compared to larval motoneurons, but no other findings supported the tested hypotheses. These results suggest that developmental weakening of the pre-ecdysis motor pattern results from changes upstream of the compression motoneurons and their synaptic connections from IN-402s. Accepted: 29 September 1999     

Dendritic properties of uropod motoneurons and premotor nonspiking interneurons in the crayfish Procambarus clarkii Girard

Dendritic properties of uropod motoneurons and premotor nonspiking interneurons of crayfish have been studied using intradendritic recording and current injection. The input resistance of phasic motoneurons (5.20 ± 0.5 M; mean ± standard error) measured by injecting constant hyperpolarizing current was significantly lower than that of tonic motoneurons (10.3 ± 2.6 M; 0.02 < P < 0.05). The membrane time constant of phasic motoneurons (7.3 ± 0.9 ms) was also significantly shorter than that of tonic motoneurons (24.3 ± 2.5 ms; P < 0.001). Both types of motoneurons behaved linearly during hyperpolarization and sub-threshold depolarization. Nonspiking interneurons showed outward rectification upon depolarization. During hyperpolarization, their membrane behaved linearly and showed significantly higher input resistance (19.5 ± 2.5 M) than phasic and tonic motoneurons (P < 0.001). Their membrane time constant (38.0 ± 5.7 ms) was significantly longer than that of phasic motoneurons (P < 0.001) but not than that of tonic motoneurons (P > 0.05). In response to intracellular injection of sinusoidally oscillating current, phasic motoneurons showed one or two spikes per depolarization period irrespective of oscillating frequency ranging from 1 to 16 Hz. Tonic motoneurons showed larger numbers of spikes per stimulus period at lower frequencies. Nonspiking interneurons also showed phase-locked effects on the motoneuron spike activity. The effective frequency range over which injected oscillating current could modulate motoneuron spike activity was similar for tonic motoneurons and nonspiking interneurons.     

Cholinergic control of the walking network in the crayfish Procambarus clarkii

The output of a neuronal network results generally from both the properties of the component neurons and their synaptic relationships. This article aims at synthesizing various results obtained on the neural network generating locomotion in vitro. In the preparation used, consisting of the last three thoracic ganglia (3–5) along with motor nerves from the 5th leg ganglion to the promotor, remotor, levator and depressor muscles, motor nerve recordings generally revealed only tonic activity in several different motoneurons (MNs). However, rhythmic activity can be obtained by the use of cholinergic agents such as the oxotremorine (Oxo) superfused in the bath (5 × 10⁻⁵ M). If Oxo is pressure-ejected locally in the ganglion, it is possible, depending upon the locus where the drug is applied, to elicit a rhythmic activity restricted to a group of antagonistic MNs. To analyze how cholinergic agents are able to induce such rhythmic activity, very small volumes of drug (50–200 pl), were applied close to the recording electrode. Two types of depolarizing response occurred: a fast large amplitude depolarization (5–20 mV) and a long lasting (10 s to several minutes) low amplitude depolarization (1–3 mV). These responses persisted in the presence of TTX and Co²⁺. The transient initial depolarization is a mixed nicotinic and muscarinic voltage-independent response during which the input resistance decreases by 20 to 40%. In contrast, the long lasting component is voltage-dependent, exclusively muscarinic and associated to a 5–10% increase of input resistance due to the closing of a K⁺ conductance that is active at the resting Vm, and totally suppressed at holding potentials below −70 mV. More generally, K⁺ currents activated at resting potential are responsible for membrane potential stability. The injection of TEA, a blocker of the K⁺ currents, through the recording electrode is able to unmask plateaus above a threshold depolarization. These plateaus are TTX-sensitive but persist in the presence of Ca²⁺ channel blockers. Moreover, in 10% of TEA-filled MNs a spontaneous pacemaker activity was revealed. The organization of the locomotor network is also based upon connections between MNs and INs. Within a MN pool, connections are only loosely established, appearing to consist mainly of electrical coupling. Inhibitory synaptic connections between MNs of opposite pools are mediated by chloride channels. However, the neurotransmitter involved could be either GABA or glutamate. Therefore, at the level of a given joint, a basic rhythm occurs due to both motoneuronal membrane properties and motoneuronal connectivity. However, the coordination of all MNs of an entire leg during fictive walking activity requires the involvement of INs. Based upon these data, we propose a two-stage model of the locomotor network organization: a joint motoneuronal level and a whole leg interneuronal level.     

Monosynaptic rabies virus reveals premotor network organization and synaptic specificity of cholinergic partition cells

Movement is the behavioral output of neuronal activity in the spinal cord. Motor neurons are grouped into motor neuron pools, the functional units innervating individual muscles. Here we establish an anatomical rabies virus-based connectivity assay in early postnatal mice. We employ it to study the connectivity scheme of premotor neurons, the neuronal cohorts monosynaptically connected to motor neurons, unveiling three aspects of organization. First, motor neuron pools are connected to segmentally widely distributed yet stereotypic interneuron populations, differing for pools innervating functionally distinct muscles. Second, depending on subpopulation identity, interneurons take on local or segmentally distributed positions. Third, cholinergic partition cells involved in the regulation of motor neuron excitability segregate into ipsilaterally and bilaterally projecting populations, the latter exhibiting preferential connections to functionally equivalent motor neuron pools bilaterally. Our study visualizes the widespread yet precise nature of the connectivity matrix for premotor interneurons and reveals exquisite synaptic specificity for bilaterally projecting cholinergic partition cells.     

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.     

Sensorimotor pathways involved in interjoint reflex action of an insect leg

Coordination of motor output between leg joints is crucial for the generation of posture and active movements in multijointed appendages of legged organisms. We investigated in the stick insect the information flow between the middle leg femoral chordotonal organ (fCO), which measures position and movement in the femur-tibia (FT) joint and the motoneuron pools supplying the next proximal leg joint, the coxa-trochanteral (CT) joint. In the inactive animal, elongation of the fCO (by flexing the FT joint) induced a depolarization in eight of nine levator trochanteris motoneurons, with a suprathreshold activation of one to three motoneurons. Motoneurons of the depressor trochanteris muscle were inhibited by fCO elongation. Relaxation signals, i.e., extension of the FT joint, activated both levator and depressor motoneurons; i.e., both antagonistic muscles were coactivated. Monosynaptic as well as polysynaptic pathways contribute to interjoint reflex actions in the stick insect leg. fCO afferents were found to induce short latency EPSPs in levator motoneurons, providing evidence for direct connections between fCO afferents and levator motoneurons. In addition, neuronal pathways via intercalated interneurons were identified that transmit sensory information from the fCO onto levator and/or depressor motoneurons. Finally, we describe two kinds of alterations in interjoint reflex action: (a) With repetitive sensory stimulation, this interjoint reflex action shows a habituation-like decrease in strength. (b) In the actively moving animal, interjoint reflex action in response to fCO elongation, mimicking joint flexion, qualitatively remained the same sign, but with a marked increase in strength, indicating an increased influence of sensory signals from the FT joint onto the adjacent CT joint in the active animal. © 1997 John Wiley & Sons, Inc. J Neurobiol 33: 891–913, 1997     

Longitudinal neuronal organization and coordination in a simple vertebrate: a continuous,semi-quantitative computer model of the central pattern generator for swimming in young frog tadpoles

When frog tadpoles hatch their swimming requires co-ordinated contractions of trunk muscles, driven by motoneurons and controlled by a Central Pattern Generator (CPG). To study this co-ordination we used a 3.5 mm long population model of the young tadpole CPG with continuous distributions of neurons and axon lengths as estimated anatomically. We found that: (1) alternating swimming-type activity fails to self-sustain unless some excitatory interneurons have ascending axons, (2) a rostro-caudal (R-C) gradient in the distribution of excitatory premotor interneurons with short axons is required to obtain the R-C gradient in excitation and resulting progression of motoneuron firing necessary for forward swimming, (3) R-C delays in motoneuron firing decrease if excitatory motoneuron to premotor interneuron synapses are present, (4) these feedback connections and the electrical synapses between motoneurons synchronise motoneuron discharges locally, (5) the above findings are independent of the detailed membrane properties of neurons.