Control of Stepping Velocity in the Stick Insect

36 We performed electrophysiological and behavioral experiments in single leg preparations 37 and intact animals of the stick insect Carausius morosus to understand mechanisms 38 underlying the control of walking speed. 39 At the level of the single leg, we found no significant correlation between stepping velocity 40 and spike frequency of motor neurons other than the previously shown modification in flexor 41 (stance) MN activity (Gabriel and Büschges 2007). However, the pause between stance and 42 swing motoneuron activity at the transition from stance to swing phase and stepping velocity 43 are correlated. This pause becomes shorter with increasing speed and completely disappears 44 during fast stepping sequences. By means of extraand intracellular recordings in single leg 45 stick insect preparations we found no systematic relationship between the velocity of a 46 stepping front leg and the motoneuronal activity in the ipsior contralateral mesothoracic 47 protractor and retractor, as well as flexor and extensor MNs. 48 The observations on the lack of coordination of stepping velocity between legs in single leg 49 preparations were confirmed in behavioral experiments with intact stick insects tethered 50 above a slippery surface, thereby effectively removing mechanical coupling through the 51 ground. In this situation, there were again no systematic correlations between the stepping 52 velocities of different legs despite the finding that an increase in stepping velocity in a single 53 front leg is correlated with a general increase in nerve activity in all connectives between the 54 subesophageal and all thoracic ganglia. However, when the tethered animal increased walking 55 speed due to a short tactile stimulus, provoking an escape-like response, stepping velocities of 56 ipsilateral legs were found to be correlated for several steps. These results indicate that there 57 is no permanent coordination of stepping velocities between legs, but that such coordination 58 can be activated under certain circumstances. 59 60 61