Effects of functional decoupling of a leg in a model of stick insect walking incorporating three ipsilateral legs

Abstract

Legged locomotion is a fundamental form of activity of insects during which the legs perform coordinated movements. Sensory signals conveying position, velocity and load of a leg are sent between the thoracic ganglia where the local control networks of the leg muscles are situated. They affect the actual state of the local control networks, hence the stepping of the legs. Sensory coordination in stepping has been intensively studied but important details of its neuronal mechanisms are still unclear. One possibility to tackle this problem is to study what happens to the coordination if a leg is, reversibly or irreversibly, deprived of its normal function. There are numerous behavioral studies on this topic but they could not fully uncover the underlying neuronal mechanisms. Another promising approach to make further progress here can be the use of appropriate models that help elucidate those coordinating mechanisms. We constructed a model of three ipsilateral legs of a stick insect that can mimic coordinated stepping of these legs. We used this model to investigate the possible effects of decoupling a leg. We found that decoupling of the front or the hind leg did not disrupt the coordinated walking of the two remaining legs. However, decoupling of the middle leg yielded mixed results. Both disruption and continuation of coordinated stepping of the front and hind leg occurred. These results agree with the majority of corresponding experimental findings. The model suggests a number of possible mechanisms of decoupling that might bring about the changes.

A kinematic model of stick‐insect walking

“…Of course, we could have tried to change the intrinsic properties of those INs and MNs to obtain the desired results but they had been used with the existing properties in several previous successful simulations (e.g., Tóth et al. , , ; Tóth and Daun ). Any change in their properties could therefore have endangered the success of those previous simulations if those simulations had been repeated with the new properties of (some of) the neurons.…”

“…The main property to be demonstrated was the simultaneous display of search movements of the front legs and the continued normal walking of the other four legs. We could achieve this goal by using the premotor INs to inhibit the depressor MNs, hence inactivate the corresponding muscles and disinhibit the levator MNs (and activate the corresponding muscles) of L1 and R1 in the model, in a similar way to what was done in Tóth and Daun (). In addition, the same mechanism in the PR system ensured a protracted position of the front legs.…”

“…Eventually, the three‐leg model could, on the ipsilateral side, produce the usual coordination patterns (tetrapod and tripod), and the transition between them (Tóth and Daun‐Gruhn ). Details of the properties and capabilities of this model can be found in the paper just cited, as well as in the papers Tóth and Daun (); Tóth et al. ().…”

“…It is more complex for the latter coordination pattern. This model was used in the simulations in which we studied the possible effects of decoupling one of the legs from the coordination mechanism of the three legs (Tóth and Daun ).…”