The control of antennal movements by leg proprioceptors in the rock lobster,Palinurus vulgaris

Summary The antennae of the rock lobster,Palinurus vulgaris, show systematic responses to movements of the legs on a tilting footboard. Myographic recordings in muscles of the first antennal segment have been used in an analysis of the sensory basis of these reactions. Antennal posture is modified in the experimental apparatus, although its relation to the change in loading conditions of the legs is uncertain. The motor control of the antennal equilibrium responses involves a complete reciprocation between both excitatory and inhibitory motoneurones to the antagonist muscle groups in the two antennae. Sensory inputs from single legs produce movements of both antennae, but a stronger drive ipsilaterally. Leg receptor inputs also modulate antennal resistance reflexes in a systematic manner, providing a sensitive test for the involvement of particular receptor organs in the leg. Movement at the coxo-basal leg joint is a major source of sensory input, and ablation/ stimulation experiments have established that stimulation of the CB chordotonal organ is a necessary but not sufficient condition to produce the antennal equilibrium reactions. The possibility is discussed that other receptors at the coxo-basal joint are also involved.D.M.N. was supported by a grant from The Max-Planck Institut to Professor H. Schöne.     

TRANSMITTER METABOLIZING ENZYMES AND FREE AMINO ACID LEVELS IN SENSORY AND MOTOR NERVES AND GANGLIA OF THE SHORE CRAB (CARCINUS MAENAS)

—The distribution of ChAT (choline acetyltransferase), GAD (glutamate decarboxylase) and acetylcholinesterase in some sensory and motor nerves of the shore crab, Carcinus maenas, has been investigated using micro-assay techniques. ChAT was concentrated in the afferent nerve fibres of the thoracic-coxal muscle receptor as well as in the coxo-basal chordotonal receptor nerve and other leg sensory fibres. GAD was found in leg motor nerves including the promotor and remotor muscle nerves, being undetectable in the sensory nerves. Acetylcholinesterase was found in similar levels in both sensory and motor nerves assayed. Amino acid analysis using a micro-dansylation technique showed that sensory nerves had low GABA levels, whereas the leg nerve including motor fibres had substantially higher GABA concentrations. GAD and GABA were also found in low amounts in the leg promoter mucle, which is consistent with GABA being a neuromuscular transmitter.     

Possible contributions of CPG activity to the control of rhythmic human arm movement

There is extensive modulation of cutaneous and H-reflexes during rhythmic leg movement in humans. Mechanisms controlling reflex modulation (e.g., phase- and task-dependent modulation, and reflex reversal) during leg movements have been ascribed to the activity of spinal central pattern generating (CPG) networks and peripheral feedback. Our working hypothesis has been that neural mechanisms (i.e., CPGs) controlling rhythmic movement are conserved between the human lumbar and cervical spinal cord. Thus reflex modulation during rhythmic arm movement should be similar to that for rhythmic leg movement. This hypothesis has been tested by studying the regulation of reflexes in arm muscles during rhythmic arm cycling and treadmill walking. This paper reviews recent studies that have revealed that reflexes in arm muscles show modulation within the movement cycle (e.g., phase-dependency and reflex reversal) and between static and rhythmic motor tasks (e.g., task-dependency). It is concluded that reflexes are modulated similarly during rhythmic movement of the upper and lower limbs, suggesting similar motor control mechanisms. One notable exception to this pattern is a failure of contralateral arm movement to modulate reflex amplitude, which contrasts directly with observations from the leg. Overall, the data support the hypothesis that CPG activity contributes to the neural control of rhythmic arm movement.     

A strand receptor with a central cell body synapses upon spiking local interneurones in the locust

Summary Movements of the femoro-tibial joint of a locust hind leg are monitored by three classes of proprioceptors; a chordotonal organ (Usherwood et al. 1968), multipolar joint receptors (Coillot and Boistel 1968) and a strand receptor innervated by a single afferent with a central cell body (Bräunig 1985). All three classes are excited by imposed or voluntary extension of the tibia. The strand receptor (fe-tiSR) spikes tonically and at a frequency dependent upon the position of the joint whilst the multipolar joint receptors give overlapping information but for a more restricted range. The afferent from the strand receptor makes an excitatory connection with a spiking local interneurone in the midline group of the metathoracic ganglion. The central latency and consistency with which the EPSP follows each sensory spike suggests that the connection is direct. This interneurone also receives convergent inputs from neurones in the chordotonal organ, but not from multipolar joint receptors. Neither the strand receptor nor the multipolar joint receptors apparently synapse upon leg motor neurones that we have tested, in contrast to receptors in the chordotonal organ.     

Responses of efferent octopaminergic thoracic unpaired median neurons in the locust to visual and mechanosensory signals

Insect thoracic ganglia contain efferent octopaminergic unpaired median neurons (UM neurons) located in the midline, projecting bilaterally and modulating neuromuscular transmission, muscle contraction kinetics, sensory sensitivity and muscle metabolism. In locusts, these neurons are located dorsally or ventrally (DUM- or VUM-neurons) and divided into functionally different sub-populations activated during different motor tasks. This study addresses the responsiveness of locust thoracic DUM neurons to various sensory stimuli. Two classes of sense organs, cuticular exteroreceptor mechanosensilla (tactile hairs and campaniform sensilla), and photoreceptors (compound eyes and ocelli) elicited excitatory reflex responses. Chordotonal organ joint receptors caused no responses. The tympanal organ (Müller's organ) elicited weak excitatory responses most likely via generally increased network activity due to increased arousal. Vibratory stimuli to the hind leg subgenual organ never elicited responses. Whereas DUM neurons innervating wing muscles are not very responsive to sensory stimulation, those innervating leg and other muscles are very responsive to stimulation of exteroreceptors and hardly responsive to stimulation of proprioceptors. After cutting both cervical connectives all mechanosensory excitation is lost, even for sensory inputs from the abdomen. This suggests that, in contrast to motor neurons, the sensory inputs to octopaminergic efferent neuromodulatory cells are pre-processed in the suboesophageal ganglion.     

Role of local nonspiking interneurons in the generation of rhythmic motor activity in the stick insect

Local nonspiking interneurons in the thoracic ganglia of insects are important premotor elements in posture control and locomotion. It was investigated whether these interneurons are involved in the central neuronal circuits generating the oscillatory motor output of the leg muscle system during rhythmic motor activity. Intracellular recordings from premotor nonspiking interneurons were made in the isolated and completely deafferented mesothoracic ganglion of the stick insect in preparations exhibiting rhythmic motor activity induced by the muscarinic agonist pilocarpine. All interneurons investigated provided synaptic drive to one or more motoneuron pools supplying the three proximal leg joints, that is, the thoraco-coxal joint, the coxa-trochanteral joint and the femur-tibia joint. During rhythmicity in 83% (n=67) of the recorded interneurons, three different kinds of synaptic oscillations in membrane potential were observed: (1) Oscillations were closely correlated with the activity of motoneuron pools affected; (2) membrane potential oscillations reflected only certain aspects of motoneuronal rhythmicity; and (3) membrane potential oscillations were correlated mainly with the occurrence of spontaneous recurrent patterns (SRP) of activity in the motoneuron pools. In individual interneurons membrane potential oscillations were associated with phase-dependent changes in the neuron's membrane conductance. Artificial changes in the interneurons' membrane potential strongly influenced motor activity. Injecting current pulses into individual interneurons caused a reset of rhythmicity in motoneurons. Furthermore, current injection into interneurons influenced shape and probability of occurrence for SRPs. Among others, identified nonspiking interneurons that are involved in posture control of leg joints were found to exhibit the above properties. From these results, the following conclusions on the role of nonspiking interneurons in the generation of rhythmic motor activity, and thus potentially also during locomotion, emerge: (1) During rhythmic motor activity most nonspiking interneurons receive strong synaptic drive from central rhythm-generating networks; and (2) individual nonspiking interneurons some of which underlie sensory-motor pathways in posture control, are elements of central neuronal networks that generate alternating activity in antagonistic leg motoneuron pools. © 1995 John Wiley & Sons, Inc.     

The walking-(and searching-) pattern generator of stick insects,a modular system composed of reflex chains and endogenous oscillators

Stick insects (Cuniculina impigra) possessing only one front leg with restrained coxa performed searching movements or walked on a treadband. The movements are described. Ablation, surgical manipulation or experimental stimulation of different sense organs (femoral chordotonal organ, campaniform sensilla on trochanter and femur basis, proprioceptors at the coxatrochanter joint) were performed, and the resulting changes in motor output were recorded. These experiments demonstrate that the walking- and searching-pattern generators cannot be separated, at least not for the movements investigated. This walking- and searching-pattern generator consists of central modules, each of which produces irregular alternation of the activity of motor neurones of antagonistic muscles of a single joint, and of reflex loops. At least some of these reflex loops are only present in the active animal. They are responsible either for the control of a single joint or for the coordination of the movements of separate joints. The performance of these reflexes does not only depend on the state of activity of the animal; some of them additionally seem to depend on the context signalled by other sense organs.     

Synaptic connections between sensory afferents and the common inhibitory motoneuron in crayfish

The sensory inputs to the common inhibitory motoneuron that innervates every leg muscle of the crayfish Procambarus clarkii (Girard) were analyzed by performing intracellular recordings from its neurite within the neuropil of the 5th thoracic ganglion. Two types of sensory inputs involved in locomotion were studied, those from a movement coding proprioceptor (the coxobasal chordotonal organ) and those from sensory neu rons coding contact forces exerted at the tip of the leg on the substrate (the dactyl sensory afferents). Sinusoidal movements applied to the chordotonal organ strand induced a stable biphasic response in the common inhibitory motoneuron that consisted of bursts of spikes during release and stretch of the strand, corresponding to raising and lowering of the leg, respectively. Using ramp movements imposed on the chordotonal strand, we demonstrated that only movement-coding chordotonal afferents produce excitatory post-synaptic potentials in the common inhibitory motoneuron; these connections are monosynaptic. Mechanical or electrical stimulation of the dactyl sensory afferents resulted in an increase in the tonic discharge of the common inhibitory motoneuron through polysynaptic excitatory pathways. These two types of sensory cues reinforce the central command of the common inhibitory motoneuron and contribute to enhancing its activity during leg movements, and thus facilitate the relaxation of tonic muscle fibres during locomotion.Abbreviations ADR anterior distal root - A Lev anterior levator nerve - CB coxo-basipodite joint - CBCO coxo-basal chordotonal organ - CI common inhibitory motoneuron - Dep depressor nerve - DSA dactyl sensory afferents - EPSP excitatory post-synaptic potential - IN interneuron - MN motoneuron - PDR posterior distal root - P Lev posterior levator nerve - Pro promotor nerve - Rem remotor nerve     

The antennal motor system of the stick insect Carausius morosus: anatomy and antennal movement pattern during walking

The stick insect Carausius morosus continuously moves its antennae during locomotion. Active antennal movements may reflect employment of antennae as tactile probes. Therefore, this study treats two basic aspects of the antennal motor system: First, the anatomy of antennal joints, muscles, nerves and motoneurons is described and discussed in comparison with other species. Second, the typical movement pattern of the antennae is analysed, and its spatio-temporal coordination with leg movements described. Each antenna is moved by two single-axis hinge joints. The proximal head-scape joint is controlled by two levator muscles and a three-partite depressor muscle. The distal scape-pedicel joint is controlled by an antagonistic abductor/ adductor pair. Three nerves innervate the antennal musculature, containing axons of 14-17 motoneurons, including one common inhibitor. During walking, the pattern of antennal movement is rhythmic and spatiotemporally coupled with leg movements. The antennal abduction/adduction cycle leads the protraction/retraction cycle of the ipsilateral front leg with a stable phase shift. During one abduction/adduction cycle there are typically two levation/depression cycles, however, with less strict temporal coupling than the horizontal component. Predictions of antennal contacts with square obstacles to occur before leg contacts match behavioural performance, indicating a potential role of active antennal movements in obstacle detection.     

A model of pattern generation of cockroach walking reconsidered

Cockroaches that have been decapitated or that have cut thoracic connectives can show rhythmic bursting in motoneurons to intrinsic leg muscles. These preparations have been studied as models for walking and to evaluate the functions of leg proprioceptors. The present study demonstrates that headless cockroaches walk extremely poorly and slowly with considerable discoordination of motoneuronal activity, these preparations show rhythmic motoneuron bursting that is similar to righting responses (attempts to turn upright) of intact animals when placed on their backs, and bursting is inhibited when a headless animal is turned or turns itself upright. Thus, rhythmic motoneuron activity of these preparations is most probably attempted righting rather than walking. It is concluded that the headless cockroach is useful for understanding the motor mechanisms underlying righting and walking but is not of value in assessing the functions of proprioceptive feedback.     

Activation of interlimb interactions increases the motor output in legs of healthy subjects: Study under the conditions of arm and leg unloading

The effect of arm movements and movements of individual arm joints on the electrophysiological and kinematic characteristics of voluntary and vibration-triggered stepping-like leg movements was studied under the conditions of horizontal support of the upper and lower limbs. The horizontal support of arms provided a significant increase in the rate of activation of locomotor automatism by noninvasive impact on tonic sensory inputs. The addition of active arm movements during involuntary stepping-like leg movements led to an increase in the EMG activity of hip muscles and was accompanied by an increase in the amplitude of hip and shin movements. The movement of the shoulder joints led to an increase in the activity of hip muscles and was accompanied by an increase in the amplitude of hip and shin movements. Passive arm movements had the same effect on induced leg movements. The movement of the shoulder joints led to an increase in the activity of hip muscles and an increase in the amplitude of movements of knee and hip joints. At the same time, the movement of forearms and wrists had a similar facilitating effect on the physiological and kinematic characteristics of rhythmic stepping-like movements, but influenced the distal segments of legs to a greater extent. Under the conditions of subthreshold vibration of leg muscles, voluntary arm movements led to activation of involuntary rhythmic stepping movements. During voluntary leg movements, the addition of arm movements had a significantly smaller impact on the parameters of rhythmic stepping than during involuntary leg movements. Thus, the simultaneous movements of the upper and lower limbs are an effective method of activation of neural networks connecting the rhythm generators of arms and legs. Under the conditions of arm and leg unloading, the interactions between the cervical and lumbosacral segments of the spinal cord seem to play the major role in the impact of arm movements on the patterns of leg movements. The described methods of activation of interlimb interactions can be used in the rehabilitation of post-stroke patients and patients with spinal cord injuries, Parkinson’s disease, and other neurological diseases.     

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     

Direct excitation of nonspiking local interneurones by exteroceptors underlies tactile reflexes in the locust

Summary Tactile stimulation of a leg of the locustSchistocerca gregaria can lead to specific reflex movements of that leg. At the same time nonspiking interneurones that are presynaptic to the participating motor neurones are excited or inhibited, suggesting that they are directly involved in these reflexes. The afferent pathways mediating these effects have been examined by recording from individual afferents and nonspiking interneurones.Afferent spikes fromtrichoid orcampaniform sensilla on specific regions of a leg evoke chemically-mediated EPSPs with a constant central latency of about 1.5 ms in certain nonspiking interneurones. The branches of an interneurone and the afferents from which it receives inputs overlap in the neuropil of the ganglion.No afferents have been found to evoke IPSPs directly in the nonspiking interneurones. Instead the inhibition is caused by a population of spiking local interneurones that are themselves excited directly by the afferents, and whose spikes evoke IPSPs in certain nonspiking interneurones.The tactile reflexes can involve movements about one or more joints of the leg, and these coordinated responses are explained by the participation of specific nonspiking interneurones that distribute the sensory inputs to the appropriate sets of motor neurones. For example, when hairs on the dorsal surface of a tarsus are touched, the tarsus is levated. This reflex involves nonspiking local interneurones which are excited directly by these hair afferents and which make direct excitatory connections with the single levator tarsi motor neurone.     

Peculiarities of Activation of Muscles of the Shoulder Belt in Voluntary Two-Joint Movements of the Upper Limb

We studied coordination of central motor commands (СMCs) coming to muscles of the shoulder and shoulder belt in the course of single-joint and two-joint movements including flexion and extension of the elbow and shoulder joints. Characteristics of rectified and averaged EMGs recorded from a few muscles of the upper limb were considered correlates of the CMC parameters. Special attention was paid to coordination of CMCs coming to two-joint muscles that are able to function as common flexors (m. biceps brachii, caput breve, BBcb) and common extensors (m. triceps brachii, caput longum, TBcl) of the elbow and shoulder joints. Upper limb movements used in the tests included planar shifts of the arm from one spatial point to another resulting from either simultaneous changes in the angles of the shoulder and elbow joints or isolated sequential (two-stage) changes in these joint angles. As was found, shoulder muscles providing movements of the elbow with changes in the angle of the elbow joint, i.e., BBcb and TBcl, were also intensely involved in the performance of single-joint movements in the shoulder joint. The CMCs coming to two-joint muscles in the course of two-joint movements appeared, in the first approximation, as sums of the commands received by these muscles in the course of corresponding single-joint movements in the elbow and shoulder joints. Therefore, if we interpret the isolated forearm movement performed due to a change in the angle of the elbow joint as the main motor event, while the shoulder movement is considered the accessory one, we can conclude that realization of a two-joint movement of the upper-limb distal part is based on superposition of CMCs related to basic movements (main and accessory). Neirofiziologiya/Neurophysiology, Vol. 41, No. 1, pp. 48–56, January–February, 2009.     

Sensory characteristics of the P afferent neurone of the crab thoracic-coxal muscle receptor organ

Intracellular recordings were made from the P fibre, the smallest of the three afferent neurones innervating the thoracic-coxal muscle receptor organ of the crab (Carcinus maenas). While the two larger afferents are nonspiking, the response of the P fibre to a trapezoidal change in receptor muscle length consists of a single action potential signalling the onset of stretch superimposed on a graded amplitude receptor potential. The P fibre is sensitive to the velocity of the applied stretch, but is insensitive to static joint position, stretch amplitude and the velocity of the release phase. The presence and amplitude of the action potential depends on the initial length of the receptor muscle, the tension caused by efferent activation of the receptor muscle prior to receptor stretch, and on the velocity of stretch. Length constant (1.9 mm) and specific membrane resistance (76 K · cm²) values obtained for the P fibre, together with its small diameter (7 m) suggest that this neurone is less well adapted to conveying passive signals to the thoracic ganglion than are the S and T fibres. It is likely that the P fibre complements the length sensitivity of the S fibre and the tension and velocity sensitivity of the T fibre by signalling the onset of receptor stretch via single action potentials.Abbreviations TCMRO thoracic-coxal muscle receptor organ - TTX tetrodotoxin     

Interlimb interactions during cyclic in-phase and antiphase movements of arms and legs and their dependence on afferent influences

Coordinated arm and leg movements imply neural interactions between the rhythmic generators of the upper and lower extremities. In ten healthy subjects in the lying position, activity of the muscles of the upper and lower extremities was recorded during separate and joint cyclic movements of the arms and legs with different phase relationships between the movements of the limbs and under various conditions of the motor task. Antiphase active arm movements were characterized by higher muscle activity than during the inphase mode. The muscle activity during passive arm movements imposed by the experimentalist was significantly lower than muscle activity during passive arm movements imposed by the other arm. When loading one arm, the muscle activity in the other, passively moving, arm increased independently from the synergy of arm movements. During a motor task implementing joint antiphase movements of both upper and lower extremities, compared to a motor task implementing their joint in-phase movements, we observed a significant increase in activity in the biceps brahii muscle, the tibialis anterior muscle, and the biceps femoris muscle. Loading of arms in these motor tasks has been accompanied by increased activity in some leg muscles. An increase in the frequency of rhythmic movements resulted in a significant growth of the muscle activity of the arms and legs during their cooperative movements with a greater rate of rise in the flexor muscle activity of the arms and legs during joint antiphase movements. Thus, both the spatial organization of movements and the type of afferent influences are significant factors of interlimb interactions, which, in turn, determine the type of neural interconnections that are involved in movement regulation.     

Activity of neuromodulatory neurones during stepping of a single insect leg

Octopamine plays a major role in insect motor control and is released from dorsal unpaired median (DUM) neurones, a group of cells located on the dorsal midline of each ganglion. We were interested whether and how these neurones are activated during walking and chose the semi-intact walking preparation of stick insects that offers to investigate single leg-stepping movements. DUM neurones were characterized in the thoracic nerve cord by backfilling lateral nerves. These backfills revealed a population of 6-8 efferent DUM cells per thoracic segment. Mesothoracic DUM cells were subsequently recorded during middle leg stepping and characterized by intracellular staining. Seven out of eight identified individual different types of DUM neurones were efferent. Seven types except the DUMna nl2 were tonically depolarized during middle leg stepping and additional phasic depolarizations in membrane potential linked to the stance phase of the middle leg were observed. These DUM neurones were all multimodal and received depolarizing synaptic drive when the abdomen, antennae or different parts of the leg were mechanically stimulated. We never observed hyperpolarising synaptic inputs to DUM neurones. Only one type of DUM neurone, DUMna, exhibited spontaneous rhythmic activity and was unaffected by different stimuli or walking movements.     

Influence of Vibration on Motor Responses in the Upper Arm Muscles under Stationary Conditions and during Voluntary and Evoked Arm Movements under Limb Unloading Conditions

Locomotion of mammals, including humans, is based on the rhythmic activity of spinal cord circuitries. The functioning of these circuitries depends on multimodal afferent information and on supraspinal influences from the motor cortex. Using the method of transcranial magnetic stimulation (TMS) of arm muscle areas in the motor cortex, we studied the motor evoked potentials (MEP) in the upper arm muscles in stationary conditions and during voluntary and vibration-evoked arm movements. The study included 13 healthy subjects under arm and leg unloading conditions. In the first series of experiments, with motionless limbs, the effect of vibration of left upper arm muscles on motor responses in these muscles was evaluated. In the second series of experiments, MEP were compared in the same muscles during voluntary and rhythmic movements generated by left arm m. triceps brachii vibration (the right arm was stationary). Motionless left arm vibration led to an increase in MEP values in both vibrated muscle and in most of the non-vibrated muscles. For most target muscles, MEP was greater with voluntary arm movements than with vibration-evoked movements. At the same time, a similar MEP modulation in the cycle of arm movements was observed in the same upper arm muscles during both types of arm movements. TMS of the motor cortex significantly potentiated arm movements generated by vibration, but its effect on voluntary movements was weaker. These results indicate significant differences in the degree of motor cortex involvement in voluntary and evoked arm movements. We suppose that evoked arm movements are largely due to spinal rather than central mechanisms of generation of rhythmic movements.     

Neurogenic pacemakers in the legs of Opiliones

ABSTRACT. After autotomy, the legs of all the species of Opiliones examined, and of a Kenyan Pholcid spider, twitched spontaneously at the femoro-patellar and tibio-basitarsal joints, for periods of up to an hour. These joints lack extensor muscles, extension being achieved at the femoro-patellar joint probably by haemolymph pressure, but at the tibio-basitarsal joint of Opiliones by a cuticular spring which can extend the joint fully. Comparable twitching activity could be evoked without autotomy if the central nervous system was burnt, or by asphyxiation. Electromyograms from the femur or tibia of an isolated twitching leg showed regular motor bursts which accompanied flexions, and sensory activity during extension. Forced movements of the joints did not perturb the rhythm of the motor bursts. An isolated proximal half of a femur could still generate the same bursting pattern whereas no other region showed this activity after its isolation. Bursts recorded in the tibia were shown to be dependent on the integrity of the femur. By stimulation of the femur with 1 -ms current pulses it was possible to reset the rhythm. Stimulation with 1-s pulses caused an acceleration or inhibition of the rhythm according to the direction of the current. Spontaneous bursts could be evoked in silent isolated legs, or in intact quiescent legs, by similar 1-s current pulses. It is postulated that the femur contains independent neurogenic pacemakers which are activated by injury current from the damaged leg nerve; they produce regular bursts of motor impulses without the interplay of proprioceptive loops, and are responsible for the movements observed.     

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.