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.     

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.     

Motor output to the protractor and retractor coxae muscles in stick insects walking on a treadwheel

ABSTRACT. The motor output to the protractor and retractor mucles moving the coxa of the middle leg of Carausius morosus was recorded from the thoracic nerves during walking on a treadwheel. The leg movements on the wheel were generally similar to those found in free-walking animals, but tripod coordination was relatively independent of period, and the coordination of the adult animal on the wheel was most closely related to that found in free-walking first instars. The activity of a common inhibitor and four excitatory axons of the retractor and an excitatory axon of the protractor were followed for 850 steps (in six animals) to give a summary of the behaviour of the different units. The motor activity is less stereotyped than that previously reported for insects. There was strong reciprocity between the antagonists, but this was not directly correlated with the forward and backward movements of the legs. The first part of the stance phase of the leg was accompanied by a strong burst in the protractor nerve and relatively little retractor activity. This was followed by the main retractor burst which occupied the last 60% of the stance phase. The results are compared with motor output records of the locust and with earlier force-plate measurements on the stick insect. It must be concluded that the mesothoracic leg initially resists forward movement of the body by the other legs during a typical walking step.     

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     

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.     

Anatomy and physiology of trochanteral campaniform sensilla in the stick insect, Cuniculina impigra

ABSTRACT. Four groups of campaniform sensilla are found on the trochanter of Cuniculina impigra Tedtenbacher (Phasmidae). One of these groups can be divided into two sub-groups. The sensilla are approximately parallel within each group or sub-group. As sensilla with parallel orientation will respond to the same direction of shear force, each group or sub-group of campaniform sensilla should act as one unit. When the coxa is fixed, activity in the nerve supplying the campaniform sensilla can be released by bending the femur forwards and backwards. The sensilla are sensitive to movement only in one direction. The investigated sensilla react to the stimulus with phasic-tonic discharge patterns. The dependence of the phasic component upon the velocity of the stimulus can be described by a power function. The tonic component depends on the amplitude of the stimulus. By mechanical stimulation of individual groups of sensilla it can be shown that at least two groups of campaniform sensilla contain units which respond to bending the femur backwards. The activity of some motor neurones can be influenced by slightly bending the leg in the horizontal plane. The levator trochanteris muscle is activated when the femur is bent forwards, and the frequency of the slow extensor tibiae motor neurone is increased when the femur is bent backwards. The reaction of both muscles is phasic. There is no detectable reaction in the protractor or the retractor of the coxa or the depressor trochanteris.     

The two groups of sensilla in the ventral coxal hairplate of Carausius morosus have different roles during walking

Abstract. The ventral coxal hairplate (cxHPv) of the stick insect Carausius morosus Br. (Phasmida: Bacteriidae) contains two morphologically distinct groups of sensilla designated as group 1 and 2 (Gl, G2). The function of these sensilla during walking was tested by selectively ablating one or both groups on one middle leg in thirty-four animals. It has previously been shown that ablation of the entire hairplate leads to two kinds of errors: the operated leg swings farther forward and the adjacent caudal leg ends its swing more to the rear relative to the operated leg. Following selective ablation of cxHPv Gl on the middle leg, the first kind of error is more pronounced, indicating that this group contributes more to limiting forward protraction during the swing. Following ablation of cxHPv G2, the second kind of error is more evident, indicating that during stance this group contributes more to the target information influencing the swing end-point of the adjacent caudal leg. These results are interpreted to reflect the phasic and phasic-tonic response characteristics of Gl and G2 hairs, respectively.     

Interruption of searching movements of partly restrained front legs of stick insects,a model situation for the start of a stance phase?

Stick insects (Cuniculina impigra) possessing only one foreleg with restrained coxa performed searching movements. A force transducer was introduced as an obstacle into the plane of movement of femur or tibia. Depending on where it was introduced and whether it was touched for the first time during an upward or a downward movement, different kinds of behaviour of the leg were released. For these different movements, the forces applied to the obstacle and the electrical activity of the depressor, levator, retractor and protractor muscles are described. In addition the alterations occurring after ablation of several sense organs including the trochanteral campaniform sensilla are mentioned. The described movements were similar to the corresponding behaviours during walking at the end of swing phase and the beginning of stance phase. Therefore there is some probability that results obtained by this experimental paradigm can also be applied to the swing-stance transition.     

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     

The proprioceptive function of a complex chordotonal organ associated with the mesothoracic coxa in locusts

Summary A system of chordotonal organs in the locust mesothorax consists of four subunits one of which connects to the coxa. Proprioceptive afferents from the scoloparia record the rotatory movements of the coxa. Mechanical stimulation of the sensory system by sinusoidal stretch or movements mimicking stretch as in natural walking of the locust elicits reflex activation of coxal motoneurones. Both assistance and resistance reflexes to imposed movements occur, but their intensity can vary from periods of suppression below firing threshold in a motoneurone to recruitment of additional motoneurones to the same muscle. It is concluded that some of these reflexes recorded in isolated preparations can also occur in freely walking animals where they should contribute to the muscular coordination of transitions between antagonistic movements.Abbreviations aCO, cCO, pCO, vCO anterior, coxal, posterior, ventral chordotonal organ - COS chordotonal organ system - pm-al postero-median to anterior-lateral     

Parallel processing of proprioceptive information in the terminal abdominal ganglion of the crayfish

The processing of proprioceptive information from the exopodite-endopodite chordotonal organ in the tailfan of the crayfish Procambarus clarkii (Girard) is described. The chordotonal organ monitors relative movements of the exopodite about the endopodite. Displacement of the chordotonal strand elicits a burst of sensory spikes in root 3 of the terminal ganglion which are followed at a short and constant latency by excitatory postsynaptic potentials in interneurones. The afferents make excitatory monosynaptic connections with spiking and nonspiking local interneurones and intersegmental interneurones. No direct connections with motor neurones were found.Individual afferents make divergent patterns of connection onto different classes of interneurone. In turn, interneurones receive convergent inputs from some, but not all, chordotonal afferents. Ascending and spiking local interneurones receive inputs from afferents with velocity thresholds from 2–400°/s, while nonspiking interneurones receive inputs only from afferents with high velocity thresholds (200–400°/s).The reflex effects of chordotonal organ stimulation upon a number of uropod motor neurones are weak. Repetitive stimulation of the chordotonal organ at 850°/s produces a small reduction in the firing frequency of the reductor motor neurone. Injecting depolarizing current into ascending or non-spiking local interneurones that receive direct chordotonal input produces a similar inhibition.     

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 coxal articulation of the insect striking leg: A comparative study

The thoraco-coxal muscles and the geometry of the leg suspension inside the prothorax were investigated in a praying mantis Mantis religiosa, an aquatic bug Ranatra linearis, and a neuropterid Mantispa lobata. Comparative observations were carried out on a cockroach Nauphoeta cinerea, a pentatomid bug Graphosoma italicum and a neuropterid Creoleon plumbeus. Adaptation of the thoraco-coxal joint to striking function was attained by different morphological structures, preadapted specifically in each insect order with respect to the articulation of the coxa, the number of muscles and the development of the endoskeleton. Adaptation provides for two main properties: maximal distance of the strike and the required rigidity of the supporting structures. In turn, these goals have given rise to some secondary problems, e.g., the versatility of the thoraco-coxal joint and the supposed coactivation of different muscle groups during the strike. J. Morphol. 236:127-138, 1998. © 1998 Wiley-Liss, Inc.     

Co-ordinating interneurones of the locust which convey two patterns of motor commands: their connexions with ventilatory motoneurones.

1. The interneurones which make widespread connexions with flight motoneurones also synapse upon ventilatory motoneurones so that in all 50 motoneurones receive synapses. They influence three aspects of ventilation; (a) the closing and opening movements of the thoracic spiracles, (b) some aspects of abdominal pumping movements and (c) the recruitment of some motoneurones controlling head pumping. 2. The two closer motoneurones of a particular thoracic spiracle receive the same excitatory synaptic inputs (EPSPs) during expiration. The EPSPs match those in appropriate flight motoneurones. 3. The closer motoneurones of each thoracic spiracle whose somata are in the pro-, meso- or metathoracic ganglia all receive the same excitatory synaptic inputs. These inputs are an adequate explanation of the pattern of spikes in the closer motoneurones. Both the slow ventilatory and fast rhythms of synaptic potentials are expressed as spikes; the slow as the overall expiratory burst of spikes and the fast as the groups of spikes within that burst. This establishes a ventilatory function for the interneurones. All thoracic closer motoneurones therefore receive the same excitatory commands which will tend to synchronize the movements of each spiracle. 4. Spiracular opener motoneurones are inhibited during expiration, their IPSPs matching the EPSPs in flight or closer motoneurones. Therefore the interneurones have reciprocal effects on the antagonistic motoneurones of the spiracles. 5. The interneurones synapse upon some motoneurones which control the pumping movements of the abdomen and which have their somata in the metathoracic or first unfused abdominal ganglion. Motoneurones in four separate ganglia therefore receive inputs from these interneurones. 6. The interneurones also synapse upon motoneurones which control an auxiliary form of ventilation, head pumping.     

Co-ordinating interneurones of the locust which convey two patterns of motor commands: their connexions with flight motoneurones.

1. Some flight motoneurones receive two superimposed rhythms of depolarizing synaptic potentials when the locust is not flying; a slow rhythm which is invariably linked to the expiratory phase of ventilation, and a fast rhythm with a period of about 50 ms which is similar to the wingbeat period in flight. 2. By recording simultaneously from groups of motoneurones, the synaptic potentials which underly these rhythms have been revealed in 30 flight motoneurones in the three thoracic ganglia. The inputs occur in elevator motoneurones and some depressors but are of lower amplitude in the latter. The inputs have not been found in leg motoneurones. 3. The rhythmic depolarizations are usually subthreshold but sum with sensory inputs to evoke spikes in flight motoneurones at intervals equal to or multiples of the wingbeat period in flight. 4. Both rhythms originate in the metathoracic ganglion and are mediated by the same interneurones. They can be adequately explained by supposing that there are two symmetrical interneurones which each make widespread connexions with left and right flight motoneurones in the three ganglia. 5. The slow rhythm is coded in the overall burst of interneurone spikes during expiration and the fast rhythm in the interval between the spikes of a burst.     

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.

     

Processing of mechanosensory information from gustatory receptors on a hind leg of the locust

Gustatory receptors (basiconic sensilla) on the legs of the desert locust, Schistocerca gregaria, are innervated by chemosensory afferents and by a mechanosensory afferent. We show, for the first time, that these mechanosensory afferents form an elaborate detector system with the following properties: 1) they have low threshold displacement angles that decrease with increasing stimulus frequency in the range 0.05–1 Hz, 2) they respond phasically to deflections of the receptor shaft and adapt rapidly to repetitive stimulation, 3) they encode the velocity of the stimulus in their spike frequency and have velocity thresholds lower than 1°/s, and 4) they are directionally sensitive, so that stimuli moving proximally towards the coxa elicit the greatest response.The mechanosensory afferents, but not the chemosensory afferents, make apparently monosynaptic connections with spiking local interneurones in a population with somata at the ventral midline of the metathoracic ganglion. They evoke excitatory synaptic potentials that can sum to produce spikes in the spiking local interneurones. Stimulation of the single mechanosensory afferent of a gustatory receptor can also give rise to long lasting depolarizations, or to bursts of excitatory postsynaptic potentials in the interneurones that can persist for several seconds after the afferent spikes. These interneurones are part of the local circuitry involved in the production of local movements of a leg. The mechanosensory afferents from gustatory receptors must, therefore, be considered as part of the complex array of exteroceptors that provide mechanosensory information to these local circuits for use in adjusting, or controlling locomotion.     

Motor patterns during kicking movements in the locust

Locusts (Schistocerca gregaria) use a distinctive motor pattern to extend the tibia of a hind leg rapidly in a kick. The necessary force is generated by an almost isometric contraction of the extensor tibiae muscle restrained by the co-contraction of the flexor tibiae (co-contraction phase) and aided by the mechanics of the femoro-tibial joint. The stored energy is delivered suddenly when the flexor muscle is inhibited. This paper analyses the activity of motor neurons to the major hind leg muscles during kicking, and relates it to tibial movements and the resultant forces.During the co-contraction phase flexor tibiae motor neurons are driven by apparently common sources of synaptic inputs to depolarized plateaus at which they spike. The two excitatory extensor motor neurons are also depolarized by similar patterns of synaptic inputs, but with the slow producing more spikes at higher frequencies than the fast. Trochanteral depressors spike at high frequency, the single levator tarsi at low frequency, and common inhibitors 2 and 3 spike sporadically. Trochanteral levators, depressor tarsi, and a retractor unguis motor neuron are hyperpolarized.Before the tibia extends all flexor motor neurons are hyperpolarized simultaneously, two common inhibitors, and the levator trochanter and depressor tarsi motor neurons are depolarized. Later, but still before the tibial movement starts, the extensor tibiae and levator tarsi motor neurons are hyperpolarized. After the movement has started, the extensor motor neurons are hyperpolarized further and the depressor trochanteris motor neurons are also hyperpolarized, indicating a contribution of both central and sensory feedback pathways.Variations in the duration of the co-contraction of almost twenty-fold, and in the number of spikes in the fast extensor tibiae motor neuron from 2–50 produce a spectrum of tibial extensions ranging from slow and weak, to rapid and powerful. Flexibility in the networks producing the motor pattern therefore results in a range of movements suited to the fluctuating requirements of the animal.