Femur-tibia Joint Research Articles

Leg kinematics and muscle activity during treadmill running in the cockroach, Blaberus discoidalis : I. Slow running

We have combined high-speed video motion analysis of leg movements with electromyogram (EMG) recordings from leg muscles in cockroaches running on a treadmill. The mesothoracic (T2) and metathoracic (T3) legs have different kinematics. While in each leg the coxa-femur (CF) joint moves in unison with the femurtibia (FT) joint, the relative joint excursions differ between T2 and T3 legs. In T3 legs, the two joints move through approximately the same excursion. In T2 legs, the FT joint moves through a narrower range of angles than the CF joint. In spite of these differences in motion, no differences between the T2 and T3 legs were seen in timing or qualitative patterns of depressor coxa and extensor tibia activity. The average firing frequencies of slow depressor coxa (Ds) and slow extensor tibia (SETi) motor neurons are directly proportional to the average angular velocity of their joints during stance. The average Ds and SETi firing frequency appears to be modulated on a cycle-by-cycle basis to control running speed and orientation. In contrast, while the frequency variations within Ds and SETi bursts were consistent across cycles, the variations within each burst did not parallel variations in the velocity of the relevant joints.

Leg kinematics and muscle activity during treadmill running in the cockroach, Blaberus discoidalis: II. Fast running.

We have combined kinematic and electromyogram (EMG) analysis of running Blaberus discoidalis to examine how middle and hind leg kinematics vary with running speed and how the fast depressor coxa (Df) and fast extensor tibia (FETi) motor neurons affect kinematic parameters. In the range 2.5-10 Hz, B. discoidalis increases step frequency by altering the joint velocity and by reducing the time required for the transition from flexion to extension. For both Df and FETi the timing of recruitment coincides with the maximal frequency seen for the respective slow motor neurons. Df is first recruited at the beginning of coxa-femur (CF) extension. FETi is recruited in the latter half of femur-tibia (FT) extension during stance. Single muscle potentials produced by these fast motor neurons do not have pronounced effects on joint angular velocity during running. The transition from CF flexion to extension was abbreviated in those cycles with a Df potential occurring during the transition. One effect of Df activity during running may be to phase shift the beginning of joint extension so that the transition is sharpened. FETi is associated with greater FT extension at higher running speeds and may be necessary to overcome high joint torques at extended FT joint angles.

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.

Role of presynaptic inputs to proprioceptive afferents in tuning sensorimotor pathways of an insect joint control network.

The femur-tibia (FT) joint of insects is governed by a neuronal network that controls activity in tibial motoneurons by processing sensory information about tibial position and movement provided by afferents of the femoral chordotonal organ (fCO). We show that central arborizations of fCO afferents receive presynaptic depolarizing synaptic inputs. With an average resting potential of -71.9 +/- 3.72 mV (n = 10), the reversal potential of these potentials is on average -62.8 +/- 2.3 mV (n = 5). These synaptic potentials occur either spontaneously or are related to movements at the fCO. They are thus induced by signals from other fCO afferents. Therefore, the synaptic inputs to fCO afferents are specific and depend on the sensitivity of the individual afferent affected. These potentials reduce the amplitude of concurrent afferent action potentials. Bath application of picrotoxin, a noncompetitive blocker of chloride ion channels, blocks these potentials, which indicates that they are mediated by chloride ions. From these results, it is concluded that these are inhibitory synaptic potentials generated in the central terminals of fCO afferents. Pharmacologic removal of these potentials affects the tuning of the complete FT control system. Following removal, the dependence of the FT control loop on the tibia position increases relative to the dependency on the velocity of tibia movements. This is due to changes in the relative weighting of the position and velocity signals in the parallel interneuronal pathways from the fCO onto tibial motoneurons. Consequently, the FT joint is no longer able to perform twig mimesis (i.e., catalepsy), which is known to rely on a low position compared to the high-velocity dependency of the FT control system.

Gain changes in sensorimotor pathways of the locust leg

Feedback systems that control the leg joints of animals must be highly flexible in adapting to different behavioural tasks. One manifestation of such flexibility is changes in the gain of joint control networks. The femur­tibia (FT) control network of the locust leg is one of the feedback systems most thoroughly studied with regard to its neural circuitry. Despite excellent information concerning network topology, however, actual gain changes and their underlying mechanisms have not yet been examined because of the marked spontaneous variations in the action of the control network for this joint. We describe a behavioural situation and a preparation in which the locust (Locusta migratoria L.) FT control network exhibits reproducible changes in gain, allowing investigation of the neuronal basis of gain control. After ('fictive') flight motor activity, the gain of resistance reflexes in the FT joint of the locust middle leg is significantly decreased, with the flexor tibiae muscles being affected more strongly than the extensor muscles. Immediately after flight motor activity, the gain may be as low as 30 % of pre-flight levels. It returns to pre-flight values in under 150 s. The decrease in gain following flight motor activity is due to a decrease in motoneurone recruitment in the resistance reflex elicited by stimulation of the appropriate mechanoreceptor, the femoral chordotonal organ. Motoneurone recruitment is changed as a result of a drastic decline in the stimulus-related synaptic input to the motoneurones, which appears to be produced exclusively at the level of the pre-motor network. Two factors led to this conclusion: first, we found no indication of changes in membrane potential or membrane conductance of the tibia flexor and extensor motoneurones; second, recording from identified pre-motor nonspiking interneurones demonstrated that these may be involved in the observed gain changes. The putative behavioural relevance is discussed.

Contributions of structure and innervation pattern of the stick insect extensor tibiae muscle to the filter characteristics of the muscle-joint system

It is shown that the low-pass filter characteristics of the muscle­joint system of the femur­tibia joint of the stick insect Cuniculina impigra result from co-contraction of the extensor and flexor tibiae muscles. The most distal region of the extensor muscle, which contains a high percentage of slow muscle fibres, is involved in this co-contraction. This conclusion results from the following evidence. (1) Inertial and friction forces do not affect the characteristics of the low-pass filter of the muscle­joint system. (2) There is some co-contraction of the extensor and flexor muscles during sinusoidal stimulation of the femoral chordotonal organ at high stimulus frequencies. Both muscles generate tonic forces that increase with increasing stimulus frequency and also increase with time from the beginning of stimulation until a plateau is reached. (3) For the extensor muscle, this tonic force is produced by its most distal portion only. (4) Electrical stimulation of the common inhibitory motoneurone (CI1) reduces the tonic force generated in this most distal portion of the extensor muscle. Therefore, CI1 stimulation reduces the amplitude of tibial movement in response to sinusoidal stimulation of the femoral chordotonal organ at stimulus frequencies below 0.5 Hz (over this frequency range, the tibial movement amplitude is a function of the force amplitude produced by the whole extensor muscle and there is no co-contraction), but at chordotonal organ stimulus frequencies of 1 Hz and above, CI1 stimulation increases the tibial movement amplitude (in this case, movement amplitude is limited by the degree of co-contraction of the extensor and flexor muscles). With repeated chordotonal organ stimulation at higher stimulus frequencies, the tibial movement amplitude steadily decreases. This must be a consequence of increasing levels of co-contraction of the extensor and flexor muscles, since at low stimulus frequencies (no co-contraction) there is no reduction in movement amplitude during repeated stimulations. It is concluded that co-contraction of the extensor and flexor tibiae muscles prevents instability in the reflex loop in spite of the high gain necessary for the generation of catalepsy. Therefore, the mechanism described can be considered to be an adaptation to the ecological niche occupied by this animal. The contribution of the distal part of the extensor muscle to this system can be switched off by the CI1 during active movements.

Correlation between muscle structure and filter characteristics of the muscle-joint system in three orthopteran insect species

In orthopteran insects, neural networks for joint control exhibit different characteristics due to behavioural specializations. We investigated whether these differences are generated purely by the neuronal networks, or whether characteristics of the muscles or joint architecture (muscle&shy;joint system) are also involved in these behavioural specializations. We compared the properties of the muscle system moving the femur&shy;tibia joint of the middle and hindleg of three species, Carausius morosus, Cuniculina impigra and Locusta migratoria. Four aspects were analysed for the tibial extensor muscle: (i) the frequency-dependence of motoneuronal activity in response to sinusoidal stimulation of the femoral chordotonal organ (fCO), (ii) the muscle structure, (iii) the innervation pattern of the muscle and (iv) the histochemical properties of the muscle fibres. These aspects were compared with the filter characteristics of the open-loop femur&shy;tibia control system and of the muscle&shy;joint system involved. Whereas in both phasmid species (Carausius morosus and Cuniculina impigra) the motoneuronal activity steadily increases with sinusoidal stimulation of the fCO in the frequency range 0.01&shy;5 Hz, in Locusta migratoria there is a decrease in motoneuronal activity between 0.01 and 0.3 Hz. The muscle structure is basically similar in all three species, as the number of singly innervated muscle fibres (supplied by the fast extensor tibiae motor neurone, FETi) decreases from proximal to distal. The number of triply innervated fibres supplied by the FETi, the slow extensor tibiae (SETi) and the common inhibitor 1 (CI1) is maximal in the middle of the muscle, and the number of dually innervated fibres (supplied by SETi, CI1) increases from proximal to distal. Differences between the locust and the two phasmid species exist in the distal portion of the muscle. The phasmid extensor tibiae muscle contains a morphologically distinct bundle of muscle fibres, not present in the locust, which is mostly dually innervated and which is larger in Cuniculina impigra. Similar results were obtained for the histochemical characterisation of the muscle fibres as revealed from their staining for myofibrillar ATPase activity. The number of histochemically identified fast fibres decreased from proximal to distal, while the number of slow fibres increased. In Carausius morosus and Locusta migratoria, the percentage of slow fibres increased by up to 60&shy;70 % at the distal end, while this increase was to almost 100 % in Cuniculina impigra. Apparently, the larger this distal region and the higher the percentage of slow, dually innervated fibres in it, the lower is the upper corner frequency (the stimulus frequency at which the joint control system produces a movement with 70 % of its maximal response amplitude) of the muscle&shy;joint system. In summary, it appears that the upper corner frequency of the open-loop system in Locusta migratoria (<0.05 Hz) results at least in part from properties of the neuronal joint control network, but in Carausius morosus (0.5&shy;1.0 Hz) and Cuniculina impigra (0.1&shy;0.2 Hz) it results from the upper corner frequency of the muscle&shy;joint system.

Distributed processing on the basis of parallel and antagonistic pathways simulation of the femur-tibia control system in the stick insect.

In inactive stick insects, sensory information from the femoral chordotonal organ (fCO) about position and movement of the femur-tibia joint is transferred via local nonspiking interneurons onto extensor and flexor tibiae motoneurons. Information is processed by the interaction of antagonistic parallel pathways at two levels: (1) at the input side of the nonspiking interneurons and (2) at the input side of the motoneurons. We tested by a combination of physiological experiments and computer simulation whether the known network topology and the properties of its elements are sufficient to explain the generation of the motor output in response to passive joint movements, that is resistance reflexes. In reinvestigating the quantitative characteristics of interneuronal pathways we identified 10 distinct types of nonspiking interneurons. Synaptic inputs from fCO afferents onto these interneurons are direct excitatory and indirect inhibitory. These connections were investigated with respect to position and velocity signals from the fCO. The results were introduced in the network simulation. The motor output of the simulation has the same characteristics as the real system, even when particular types of interneurons were removed in the simulation and the real system.

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.

Description of Sublethal Injuries Caused to the Colorado Potato Beetle (Coleoptera: Chrysomelidae) by Propane Flamer Treatment

A propane flamer can be used to reduce the number of adult Colorado potato beetles, Leptinotarsa decemlincata (Say), in the spring. Sublethal injuries caused to adult Colorado potato beetles by a propane flamer are described. Beetles surviving the propane flamer treatment suffered leg and antenna injuries. Segments of the appendages with the smallest diameter were damaged first. Tarsi, antennae, femur-tibia joints, and coxa-body joints were damaged sequentially. Laboratory experiments demonstrated that injuries resulted from degeneration of muscles of the different segments at temperatures in the range of 65-75°C. Beetles did not recover from these injuries.

Movement of joint angles in the legs of a walking insect, Carausius morosus

This paper provides a quantitative description of the leg movement of a stick insect walking in three different situations, namely walking tethered on a treadwheel, on a slippery glass plate and walking free on a horizontal plane. These descriptions concern the trajectories of the end of the tibia and the changes of the femur-tibia joint (γ), the coxa-trochanter joint (β) and the movement of the thoracic-coxal joint of all three pairs of legs. As the latter joint comprises a ball and socket joint, the changes of the position of the rotational axis of this joint have been measured. No simple rule could be found which allowed the control of more than one joint of a leg by a common control parameter. Co-ordination of the joints of one leg seems to be simpler during the swing than during the stance movement. Measurements of the forces at the end of the swing movement indicate that there is no information transfer between legs with respect to an expected height of the ground at the end of the swing movement, nor is there an internal “expectation” which may have led to a decrease of force before probable ground contact.

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.

Nonspiking local interneurons in insect leg motor control. I. Common layout and species-specific response properties of femur-tibia joint control pathways in stick insect and locust

1. Locusts (Locusta migratoria) and stick insects (Carausius morosus) exhibit different strategies for predator avoidance. Locusts rely primarily on walking and jumping to evade predators, whereas stick insects become cataleptic, catalepsy forming a major component of the twig mimesis exhibited by this species. The neuronal networks that control postural leg movements in locusts and stick insects are tuned differently to their specific behavioral tasks. An important prerequisite for the production of catalepsy in the stick insect is the marked velocity dependency of the control network, which appears to be generated at the level of nonspiking local interneurons. We examined interneuronal pathways in the network controlling the femur-tibia joint of the locust middle leg and compared its properties with those described for the stick insect middle leg. It was our aim to identify possible neural correlates of the species-specific behavior with regard to postural leg motor control. 2. We obtained evidence that the neuronal networks that control the femur-tibia joints in the two species consist of morphologically and physiologically similar--and thus probably homologous--interneurons. Qualitatively, these interneurons receive the same input from the femoral chordotonal organ receptors and they drive the same pools of leg motoneurons in both species. 3. Pathways that contribute to the control of the femur-tibia joint include interneurons that support both "resisting" and "assisting" responses with respect to the motoneuron activity that is actually elicited during reflex movements. Signal processing via parallel, antagonistic pathways therefore appears to be a common principle in insect leg motor control. 4. Differences between the two insect species were found with regard to the processing of velocity information provided by the femoral chordotonal organ. Interneuronal pathways are sensitive to stimulus velocity in both species. However, in the locust there is no marked velocity dependency of the interneuronal responses, whereas in the same interneurons of the stick insect it is pronounced. This characteristic was maintained at the level of the motoneurons controlling the femur-tibia joint. Pathways for postural leg motor control in the locust thus lack an important prerequisite for the generation of catalepsy, that is, a marked velocity dependency.

Peptidergic innervation of leg muscles of the cockroach, Periplaneta americana (L.), and a possible role in modulation of muscle contraction.

FMRFamide-related peptides of insects are particularly important because of their possible function as neurohormones and neuromodulators on a wide variety of tissues. Part of this study was an investigation of the immunofluorescent staining of motor nerves which arise in the metathoracic ganglion, examined in wholemount using an antiserum that recognizes extended -RFamide peptides (generally recognized to be of the FMRFamide family). This antiserum revealed immunochemical staining of numerous cell bodies in the metathoracic ganglion and of axons in peripheral nerve 5, a large nerve which contains both motor and sensory fibres. Axons staining positive for FMRFamide-related peptides were traced in nerve 5 as far as the femur-tibia joint, and into the first (sensory-motor) and third (motor only) ramus of nerve 5. Reverse-phase HPLC with radioimmunoassay revealed a peak of FMRFamide-related peptide activity in nerve 5 that was coincident with a peak found when thoracic ganglia were processed in the same fashion. A physiological assay was devised to test the ability of various non-native peptides to alter the characteristics of contraction of skeletal muscles of the legs. Using neurally evoked contractions of coxal depressor muscles of the metathoracic leg it was determined that several non-native peptides could potentiate muscle contractions. The results of this study suggest that muscles of the legs receive innervation by identifiable, FMRFamide-related peptide-containing neurons and that the release of peptide(s) at the muscle may be yet another method of modulating the mechanics of muscle contraction.

Functional morphology of the subgenual organ of the carpenter ant

Using light microscopy, confocal microscopy, electron microscopy and histochemistry, the subgenual organ (SGO) of an ant, Camponutas ligniperda, is investigated. Sensory units and attachment cells together enclose a large extracellular cavity, which is filled by acid mucopolysaccharides, as revealed by staining with ruthenium red. Due to this cavity, the whole SGO has the shape of a deformed sphere and the scolopidia exhibit a distribution of angles between 0 ° and 60 ° with the tibial long axis (as is shown by phalloidin-rhodamin staining of the actin filaments of the scolopale, viewed in situ by laser scanning confocal microscopy). The subgenual organ is innervated by a branch of the tibial nerve, which splits within or shortly distal to the femur-tibia joint. The other features of the SGO of Camponotus ligniperda are similar as in other insects: the SGO of Camponotus ligniperda contains about 35 scolopidial sensilla; it is fixed to the subgenual nerve on its proximal end, by its attachment cells to the opposite part of the cuticle; the fixation by the attachment cells is accomplished by a vast quantity of cytoplasmic microtubules; the construction of the sensory units is the same as in other mononematic scolopidial organs. The role of the extracellular lumen inside the organ and the special shape of the SGO of Camponotus ligniperda in mechanical transmission is discussed.

Intersegmental and local interneurons in the metathorax of the stick insect Carausius morosus that monitor middle leg position

1. In the stick insect, proprioceptive information from the middle leg is used to define the target for the swing movement of the adjacent rear leg ("targeting behavior"). To investigate the underlying neural circuits, intracellular recordings were made in the ganglion controlling the rear leg, the metathoracic ganglion, while systematically moving the tarsus of the middle leg. 2. Several intersegmental interneurons and one local interneuron were identified as possible contributors to the targeting behavior. The intersegmental interneurons code the position of the middle leg tarsus in a highly simplified manner: test movements of the middle leg in the dorsal, lateral, and caudal directions from the standard starting position at right angles to the thorax elicit phasic-tonic responses in three different intersegmental neurons. The response in each interneuron actually reflects the movement and position at only one joint of the middle leg: for the neurons responding primarily to movement in the caudal, dorsal, and lateral test directions, the adequate stimulus is movement at the subcoxal joint, the coxa-trochanter joint, and the femur-tibia joint, respectively. 3. The metathoracic local interneuron integrates information from ipsilateral middle and rear legs in such a way as to provide an approximate measure of the distance between the two tarsi in the longitudinal direction. It is depolarized in a phasic-tonic manner both by caudal movements of the ipsilateral middle leg and by rostral movements of the ipsilateral rear leg. The adequate stimulus in each case is the change in the angle at the subcoxal joint of the leg moved. Depolarization of this neuron activates retractor motoneurons, which is consistent with a role in terminating the swing movement. 4. Altogether the results indicate first, that the targeting behavior could be controlled by very few intersegmental channels and, second, that the nervous system encodes the position of the middle leg tarsus in terms of joint angles rather than in abstract, body-centered coordinates.

Gain control in a proprioceptive feedback loop as a prerequisite for working close to instability

In the artificially closed femur-tibia control system of stick insects oscillations were induced in 3 different ways: Increasing the phase-shift by introducing an electronic delay, afference sign reversal and coupling the tibia to an inert mass. In all 3 cases the oscillations stopped after some time. The gain of the open-loop system was significantly smaller after the oscillations. Afference sign reversal by surgically crossing of the receptor apodeme of the femoral chordotonal organ for 25–85 days does not lead to altered characteristics of the control loop. When sinusoidal passive movements are forced upon the intact femur-tibia joint the forces resisting these movements do not decrease with time. In contrast to direct stimulation of the femoral chordotonal organ, these passive movements also influence the contralateral leg. The experiments show that the gain-control system of the femur-tibia control loop of stick insects consists of at least two components: A sensitization system (with inputs from many kinds of stimuli indicating some kind of disturbance) increases the gain of all reflex loops. A specific habituation-like system decreases the gain with repetitive stimulation only of one control system.

THE PHYSIOLOGY OF SENSORY CELLS IN THE VENTRAL SCOLOPARIUM OF THE STICK INSECT FEMORAL CHORDOTONAL ORGAN

The leg joints of invertebrates are governed by neural control loops that control their position and velocity during movements (for reviews, see Bassler, 1983, 1993). These neural control loops rely on sensory feedback about the position and velocity of the controlled leg joint. In invertebrates, this sensory feedback is provided by external (e.g. hair fields, hair rows) and/or internal sense organs (e.g. chordotonal organs). The femoral chordotonal organ (fCO) serves as the main proprioceptor in the control loop governing the femur-tibia (FT) joint of the insect leg. The fCO measures the position and movement of this joint (e.g. Bassler, 1965, 1993; Burns, 1974; Usherwood et al. 1968; Zill, 1985). Previous investigations have described the physiology of sensory cells within femoral chordotonal organs (e.g. stick insect, Hofmann et al. 1985; Hofmann and Koch, 1985; locust, Matheson, 1990; Matheson and Field, 1990). Numerous investigations have been undertaken into the central processing of sensory information provided by the fCO to gain an insight into the control of FT joint movement during different behavioural tasks, for example during resistance reflexes in the standing animal (locust, Burrows, 1987, 1988; Burrows et al. 1988; stick insect, Bassler, 1988; Buschges, 1989, 1990; Driesang and Buschges, 1993) or during active movements (stick insect, Bassler, 1988; Bassler and Buschges, 1990). Most previous studies have not, however, taken into account the morphological separation of the fCO into two distinct scoloparia in the legs of some species (stick insect, Fuller and Ernst, 1973; Hofmann et al. 1985; Hofmann and Koch, 1985; locust middle leg, Burns, 1974). It has been inferred that the whole fCO supplies position and velocity information about the FT joint. In contrast, recent studies of leg reflexes have shown that only its smaller scoloparium (Fig. 1A), containing approximately one-sixth of the total number of sensory neurones, provides the sensory information that is used by the FT control loop (locust, Field and Pfluger, 1989; stick insect, Kittmann and Schmitz, 1992). These studies did not show what types of sensory neurones are located in the ventral part of the fCO and thus contribute to the FT control loop. We have therefore investigated the physiology of sensory neurones that are located in the ventral scoloparium of the fCO.

THE POSITION-DEPENDENT NATURE OF POSTURAL RESISTANCE REFLEXES IN THE LOCUST

The resistance reflexes of tibial extensor and flexor muscles, of all six legs in the locust, show changes in gain at different femur&shy;tibia angles (FTA). In all muscles the gain is high for small angles (near full flexion) and low for large angles (near full extension, approximately 160 &deg;). An exception occurs in the mesothoracic flexor muscle, which shows two modes: one as above and another in which maximum gain occurs at 100&shy;120 &deg; FTA. The position-dependent character is evident at each stage of the reflex pathway: motor neurone, non-spiking interneurone and femoral chordotonal organ (the afferent source of the reflex). We conclude that position-dependency originates from a decrease in the number of phasic femoral chordotonal organ neurones sensitive to joint movement as larger FTAs are approached. Position-dependency is only roughly correlated with the postural FTAs most commonly observed in the meso- and metathoracic legs of unrestrained resting locusts; no such correlation was evident for prothoracic legs. We propose that the major role of position-dependency is to allow resistance reflexes efficiently to counter torque introduced onto the femur&shy;tibia joint by perturbations in the horizontal plane when the insect rests on a horizontal substratum and in the vertical plane when it rests on a vertical substratum.

Octopamine effects mimick state-dependent changes in a proprioceptive feedback system.

The modulatory actions of the biogenic amine octopamine on the femur tibia (FT) control loop in the stick insect Carausius morosus were examined. The response properties of the FT control loop were determined under open loop conditions. Mechanical stimulation of the femoral chordotonal organ (fCO) was the input and tibial movement and motoneuronal activity were measured as the output of the system. Following octopamine injection into the hemolymph of intact, inactive animals, two consecutive phases occurred at the behavioral level. Octopamine caused initially an activation of the animal. During this first phase (3.5-12 min duration) the response properties of the FT control loop were similar to those found in animals that were activated by tactile stimuli under normal conditions. Afterward, animals became inactive. During this second phase (15-20 min duration), the gain of the control loop was zero and no resistance reflex in the FT joint was generated in response to fCO stimulation. However, active movements of the tibia could still be elicited. As we could show in restrained animals, where DL-octopamine was applied topically onto the undesheated mesothoracic ganglion, the complete suppression of the resistance reflex on the motoneuronal level was dose dependent starting at concentrations of 5 x 10(-3) M octopamine. We could show that octopamine specifically suppressed the pathways involved in the resistance reflex, while feedback loop responses to fCO stimuli typical for active animals could still be elicited. Our results indicate that an increase in the octopamine concentration mimicks activation of the animal: Properties being characteristic for the control of the FT joint in the inactive animal are inhibited by octopamine, while properties of the FT control loop typical for the active animal appear to be facilitated following octopamine injection. The results clearly demonstrate that different pathways in the neuronal network underlying the FT control loop are involved in the responses of the control loop to fCO stimuli in the inactive and active behavioral states of the stick insect.