The Treading-on-tarsus Reflex in Stick Insects: Phase-dependence and Modifications of the Motor Output during Walking

In stick insects walking forwards or backwards, a treading-on-tarsus (TOT) reflex of the middle leg could be elicited by mechanical stimulation of the ipsilateral front leg tarsus. The probability of eliciting a TOT reflex in the middle leg depended on the phase of the stimulus within the middle leg's step cycle and on the position of the middle leg relative to its normal anterior and posterior extreme positions. During a TOT reflex the functional swing muscle (protractor coxae in forward walks and retractor coxae in backward walks) was inhibited while the functional stance muscle was activated. This suggests that the step generator of the middle leg contains two hierarchical levels: the higher level would determine the timing of swing and stance independent of walking direction; the lower one would drive the appropriate muscle groups for swing and stance depending on which programme forward or backward walking was running. The stimulus information from the front leg necessary to elicit a TOT reflex was observed only in the ipsilateral connective. In terms of the model, this information would have to be processed according to the actual state of the higher level of the step generator. Movement and position of the front leg and tarsal contact of the middle leg were not relevant for the TOT reflex of the middle leg. Steps with a TOT reflex were significantly prolonged, and compensation for this disturbance of walking coordination was made within the next step.