Leg Coordination during Walking in Insects

Locomotion depends on constant adaptation to different requirements of the environment. An appropriate temporal and spatial coordination of multiple body parts is necessary to achieve a stable and adapted behavior. Until now it is unclear how the neuronal structures can achieve these meaningful adaptations. The exact role of the nervous system, muscles and mechanical constrains are not known. By using preparations in which special forms of adaptations are considered under experimental conditions that selectively exclude external influences, like mechanical interactions through the ground or differences in body mass, one can draw conclusions about the organization of the respective underlying neuronal structures. In the present thesis, four different publications are presented, giving evidence of mechanisms of temporal or spatial coordination of leg movements in the stick insect Carausius morosus and the fruit fly Drosophila melanogaster during different experimental paradigms. At first, state dependent local coordinating mechanisms are analyzed. Electromyographic measurements of the three major antagonistic leg muscle pairs of the forward and backward walking stick insect are evaluated. It becomes evident that only the motor activity of the most proximal leg joint is changed when walking direction is changed from forward to backward, which demonstrates that the neuronal networks driving movement in each individual leg seem to be organized in a modular structure. In the second part mechanisms that influence movement speed of the individual leg and coordination of speed between the different legs of the stick insect come into focus. Electrophysiological and behavioral experiments with the intact and reduced stick insect were used to examine relationships between the velocity of a stepping front leg and neuronal activity in the mesothoracic segment as well as correlations between the stepping velocities of different legs during walks with constant velocity or with distinct accelerations. It was shown that stepping velocity of single legs were not reflected in motoneuron activity or stepping velocity of another leg. Only when an increase in walking speed was induced, clear correlation in the stepping velocities of the individual legs was found. Subsequently, the analysis of changes in temporal leg coordination during different walking speeds in the fruit fly reveals that the locomotor system of Drosophila can cover a broad range of walking speeds and seems to follow the same rules as the locomotor system of the stick insect. Walking speed is increased by modifying stance duration, whereas swing duration and step amplitude remain largely unchanged. Changes in inter-leg coordination are gradually and systematically with walking speed and can adapt to major biomechanical changes in its walking apparatus. In the final part it was the aim to understand the role of neuronal mechanisms for the orientation and spatial coordination of foot placement in the stick insect. Placement of middle and hind legs with respect to the position of their respective rostrally neighboring leg were analyzed under two different conditions. Segment and state dependent differences in the aiming accuracy of the middle and hind legs could be shown, which indicate differences in the underlying neuronal structures in the different segments and the importance of movement in the target leg for the processing of the position information. Taken together, common principles in inter-leg coordination where found, like similarities between different organisms and segment specific or state dependent modifications in the walking system. They can be interpreted as evidence for a highly adaptive and modular design of the underlying neuronal structures

Coupling mechanisms between the contralateral legs of a walking insect (Carausius morosus)

Cruse H, Knauth A. Coupling mechanisms between the contralateral legs of a walking insect (Carausius morosus). The journal of experimental biology. 1989;144(1):199-213.Interactions between contralateral legs of stick insects during walking were examined in the absence of mechanical coupling between the legs by studying animals walking on a horizontal plane covered with a thin film of silicone oil. Investigations of undisturbed walks showed that contralateral coupling is weaker han ipsilateral coupling. Two types of influence were found, (i) For each pair of front, middle and rear legs, when one leg started a retraction movement, the probability for the contralateral leg to start a protraction was increased, (ii) For front- and hind-leg pairs, it was found that the probability of starting a protraction in one leg was also increased, the farther the other leg was moved backwards during retraction. Whether such influences exist between middle legs could not be determined. Both ‘excitatory’ mechanisms very much resemble those influences which have been found to exist between ipsilateral legs. However, in contrast to ipsilateral legs, the interaction between two contralateral legs was found to act in both directions

Peripheral influences on the movement of the legs in a walking insect Carausius morosus

Cruse H, Epstein S. Peripheral influences on the movement of the legs in a walking insect Carausius morosus. The Journal of Experimental Biology. 1982;101(1):161-170

Biologically – Plausible Load Feedback from Dynamically Scaled Robotic Model Insect Legs

Researchers have been studying the mechanisms underlying animal motor control for many years using computational models and biomimetic robots. Since testing some theories in animals can be challenging, this approach can enable unique contributions to the field. An example of a system that benefits from this modeling and robotics approach is the campaniform sensillum (CS), a kind of sensory organ used to detect the loads exerted on an insect\u27s legs. The CS on the leg are found in groups on high-stress areas of the exoskeleton and have a major influence on the adaptation of walking behavior. The challenge for studying these sensors is recording CS output from freely walking insects, which would show what the sensors detect during behavior. To address this difficulty, 3 dynamically scaled robotic models of the middle leg of the stick insect Carausius morosus (C. morosus) and the fly Drosophila melanogaster (D. melanogaster) were constructed. Two of the robotic legs model the C. morosus and are scaled to a stick insect at a ratio of 15:1 and 25:1. The robotic fly leg is scaled 400:1 to the leg of the D. melanogaster. Strain gauges are affixed to locations and orientations that are analogous to those of major CS groups. The legs were attached to a linear guide to simulate weight and they stepped on a treadmill to mimic walking. Using these robotic models, it is possible to shed light on how the nervous system of insects detects load feedback, examine the effect of different tarsi designs on load feedback, and compare the CS measurement capabilities of different insects. As mentioned earlier, robotic legs allow for any experiment to be conducted, and strain data can still be recorded, unlike animals. I subjected the 15:1 stick leg to a range of stepping conditions, including various static loading, transient loading, and leg slipping. I then processed the strain data through a previously published dynamic computational model of CS discharge. This demonstrated that the CS signal can robustly signal increasing forces at the beginning of the stance phase and decreasing forces at the end of the stance phase or when the foot slips. The same model leg can then be further expanded upon, allowing us to test how different tarsus designs affect load feedback. To isolate various morphological effects, these tarsi were developed with differing degrees of compliance, passive grip, and biomimetic structure. These experiments demonstrated that the tarsus plays a distinct role in loading the leg because of the various effects each design had on the strain. In the final experiment, two morphologically distinct insects with homologous CS groups were compared. The 400:1 robotic fly middle leg and the 25:1 robotic stick insect middle leg were used for these tests. The measured strains were notably influenced by the leg morphology, stepping kinematics, and sensor locations. Additionally, the sensor locations were lacking in one species in comparison to the other measured strains that were already being measured by the present sensors. These findings contributed to the understanding of load sensing in animal locomotion, effects of tarsal morphology, and sensory organ morphology in motor control

The control of walking movements in the leg of the rock lobster

Cruse H, Clarac F, Chasserat C. The control of walking movements in the leg of the rock lobster. Biological Cybernetics. 1983;47(2):87-94

Mechanisms for intersegmental leg coordination in walking stick insects

For efficient locomotion, the movements of single legs need to be coordinated during walking, which results in a stepping pattern or gait. This dissertation explores the neural mechanisms underlying the formation of a gait, in particular the neural basis of coupling of ipsilateral leg movements. In a semi-intact walking preparation of the stick insect Carausius morosus, correlations between ipsilateral mesothoracic motoneuron activity and walking movements of a front leg were described. Extracellular recordings showed a dedicated coupling of activity for mesothoracic protractor coxae and extensor tibiae motoneurons. Depressor trochanteris motoneurons showed a more flexible coupling. Mesothoracic retractor coxae and levator trochanteris motoneurons were active in anti-phase with their respective antagonists. Intracellular recordings revealed two different modulations of membrane potentials of mesothoracic motoneurons: a tonic modulation, lasting during the stepping activity of the front leg, and a rhythmic modulation, correlated with individual steps of the front leg. Evidence for tonic excitatory and inhibitory, as well as for rhythmic excitatory and inhibitory inputs were found for different motoneurons. Intracellular recordings of mesothoracic non-spiking interneurons of the pre-motor network revealed that these interneurons receive intersegmental coordinating signals. A tonic as well as a rhythmic modulation of their membrane potential, correlated with the walking activity of the ipsilateral front leg, were found. The non-spiking interneurons were in part morphologically identified and are known to process local sensory information. Hence, they could provide the basis for integration of local sensory and intersegmental signals. Additionally experiments were performed to investigate the origin of intersegmental signals. In experiments with an isolated chain of ganglia which was pharmacologically activated with pilocarpine, interaction between the central rhythm generating networks were studied. Sensory input was excluded in this preparation. No evidence was found for strong coupling of central pattern generators in mesothoracic and metathoracic segments, nor in prothoracic and mesothoracic segments. Two more sets of experiments focused on the role of sensory signals for intersegmental coordination. Signals from the mesothoracic femoral chordotonal organ, measuring position and movement of the femur-tibia joint, showed no clear influence on the activity of metathoracic motoneurons in the 'active' animal. Sensory signals from the metathoracic campaniform sensilla, measuring load on the leg, showed only a weak intersegmental influence on mesothoracic motoneuron activity, but a clear influence on local protractor and retractor motoneuron activity. The latter was found in the resting animal and with reversed effects in the 'active' animal, as well as during rhythmic activity evoked by application of pilocarpine

Intersegmental influences contributing to coordination in a walking insect

Locomotion depends on correct interaction of the nervous system, muscles and environment. A key element in this process is the coordinated interplay of multiple body parts to achieve a stable and adapted behavior. Different aspects of intersegmental coordination in the stick insect have been investigated in this thesis: the activation of the walking system, intersegmental information transfer in the connectives and the in uence of load signals. I used a reduced preparation with only single intact front, middle or hind legs. The intact leg(s) performed stepping movements on a passive treadmill, hence providing, both sensory feedback and central input from its active pattern generating networks to the other hemiganglia. The activity of protractor and retractor motoneurons (MNs) was simultaneously recorded extracellularly in the other segments. The preparation allows investigating intersegmental influence of stepping single leg(s) on motoneural activity in the other deafferented hemisegments. The experiments revealed that the stick insect walking system is constructed in a modular fashion. Stepping of a single leg does not imply that the animal is in a locomotor state. In the two leg preparation with two intact legs that stepped on two separate treadmills, stepping of one leg did not imply stepping of the second leg. The legs stepped independent of each other concerning coordination and frequency. In the single leg preparation stepping of a single leg did not activate pattern generating networks in all other hemiganglia. The different hemiganglia were obviously activated independently. Only forward stepping of the front leg and, to a lesser extend, backward stepping of the hind leg, elicited alternating activity in mesothoracic protractor and retractor MNs. Motoneural activity in the other hemisegments increased and was slightly modulated during stepping sequences. Activation of the metathoracic ganglion required both ipsilateral front and middle legs stepping. Furthermore, the stick insect walking system is constructed asymmetrically on the neural level concerning the contribution and importance of the different legs for intersegmental coordination. The influence of middle leg stepping was qualitatively different to the influence of front leg stepping. In the single leg preparation front leg stepping induced alternating activity in ipsilateral mesothoracic protractor and retractor MNs that was most probably shaped by pattern generating networks. Middle leg stepping did not induce alternating activity in MNs of its ipsilateral neighboring segments. In a two leg preparation with front and ipsilateral middle leg stepping the middle leg appears to have no influence on the timing of metathoracic motoneural activity whereas front leg stepping was able to entrain metathoracic MN activity. The processing of intersegmental signals from other stepping legs appears to depend on the state of the receiving ganglion. Signals from the stepping front leg most probably reach the metathoracic ganglion as connective recordings show. If the metathoracic ganglion is active in the sense that the central pattern generating networks are active the signals from a stepping leg are treated differently. If the metathoracic ganglion was not active a general increase in motoneural activity was observed during front leg stepping. In case of an active metathoracic ganglion protractor and retractor MN activity alternated and was influenced by front leg stepping. Sensory signals are particularly important for coordination of the legs in the stick insect. In experiments in which middle leg campaniform sensilla were stimulated during single front leg stepping sequences, mesothoracic levator and depressor motoneuron activity was coupled to the campaniform sensilla stimulation. Stimulation of middle campaniform sensilla pretends increased load on the leg and induced an increase in depressor and a decrease in levator motoneuron activity. In mesothoracic protractor and retractor motoneurons front leg stepping induced alternating activity. Depending on the phase of front leg step cycle middle leg campaniform sensilla stimulation increased retractor and decreased protractor motoneuron activity or the influence was reverse (around 180° of step cycle)

Universal features in panarthropod inter-limb coordination during forward walking

Terrestrial animals must often negotiate heterogeneous, varying environments. Accordingly, their locomotive strategies must adapt to a wide range of terrain, as well as to a range of speeds in order to accomplish different behavioral goals. Studies in \textit{Drosophila} have found that inter-leg coordination patterns (ICPs) vary smoothly with walking speed, rather than switching between distinct gaits as in vertebrates (e.g., horses transitioning between trotting and galloping). Such a continuum of stepping patterns implies that separate neural controllers are not necessary for each observed ICP. Furthermore, the spectrum of \textit{Drosophila} stepping patterns includes all canonical coordination patterns observed during forward walking in insects. This raises the exciting possibility that the controller in \textit{Drosophila} is common to all insects, and perhaps more generally to panarthropod walkers. Here, we survey and collate data on leg kinematics and inter-leg coordination relationships during forward walking in a range of arthropod species, as well as include data from a recent behavioral investigation into the tardigrade \textit{Hypsibius exemplaris}. Using this comparative dataset, we point to several functional and morphological features that are shared amongst panarthropods. The goal of the framework presented in this review is to emphasize the importance of comparative functional and morphological analyses in understanding the origins and diversification of walking in Panarthropoda