In the course of this thesis, neural mechanisms underlying the generation of single leg stepping in the stick insect Carausius morosus were investigated at the premotor level. Local nonspiking interneurons (NSIs) are important premotor elements within the leg muscle control system of insects, which integrate sensory signals from different sources and provide synaptic drive onto motoneurons (MNs). The single middle leg preparation used allows intracellular recordings from identified NSIs while the active animal performs stepping movements on a treadmill. For identification, NSIs were stained following physiological characterization by iontophoretical dye injection and viewed with a confocal laser scanning microscope. The alternating activity of flexor and extensor tibiae MNs during single middle leg stepping, which characterizes stance and swing phase, respectively, was monitored by extracellular recordings. In the first part of the thesis, the activity pattern of NSIs driving tibial MNs during single leg stepping was studied and their contribution to the generation of stepping motor output was revealed. With the initiation of stepping, modulations of membrane potential were generated in all NSIs that were closely related to the step cycle. The activity pattern comprised distinct excitatory or inhibitory phasic input, during at least one phase of the step cycle. Most NSI types showed an inversion of membrane potential polarization from one phase of the step cycle to the other. It was shown that the activity pattern of the individual NSIs during stepping was not predictable from the synaptic drive, i.e., excitatory or inhibitory, they provide onto MNs in the resting animal. Artificial alterations of membrane potential and measurements of local input resistance for individual NSIs revealed that phasic excitatory and inhibitory modulations of membrane potential during stepping results from true excitatory and inhibitory synaptic input. Current injections into NSI I1 immediately terminated stepping sequences, indicating an important role of I1 in the control of motor output for stepping. The amplitude of phasic membrane potential modulation of NSIs during stepping varied markedly. The maximum peak-to-peak amplitude of membrane potential modulation during stepping amounted to 16.9 ± 6.0 mV on average for all NSIs presented in this study and ranged from 5 to 34 mV for individual recordings. The time of peak and trough potential occurrence within a step cycle appears to contribute substantially to the patterning of motor output, since the extensor MN activity was closely correlated with the membrane potential of individual NSIs, e.g., E2/3, E4, E8 and I2. For the first time, it could be shown that the activity of NSIs during stepping can largely be explained by the state dependency of their inputs from the femoral chordotonal organ, one of the main leg sensors. Hence, the results presented here strongly support the notion that the motor response during the �active reaction� represents a part of the control regime for the generation of single leg stepping. In the second part of the thesis, the interest was to investigate neural mechanisms underlying adaptivity in locomotor systems. Therefore, it was examined which parameters contribute to alterations in stepping velocity. An important finding was that stepping velocity varies with membrane potential alterations of NSIs activated during stance phase, but not with NSIs activated during swing phase. Furthermore, the results suggest that the stance part of the locomotor network is stronger activated during fast stepping velocities and that the swing part is simultaneously inhibited to the same extent. However, investigation of extensor MN activity failed to show a correlation with stepping velocity. This finding implies that swing phase activity is independent of stepping velocity and, hence, corroborates the notion that the swing part of the premotor network does not contribute to alterations in stepping velocity. Finally, it was investigated whether there is a correlation between swing phase activation and stance phase velocity during single leg stepping. The results indicate that there is no influence between stance and swing phase activation in the single middle leg preparation, at least, not in the way that activation strength of stance would influence the subsequent activation of swing phase. The insights gained on premotor NSIs within the femur-tibia joint control system of the stick insect raise the assumption of a premotor network organized into functionally different and partly overlapping pools of NSIs. In the single middle leg preparation, individual NSI types appear to control the actual magnitude of stepping motor output (e.g., E2/3, E8, I2) or the stepping velocity (e.g., E1, I1, I2), while others seem to control step phase transitions (e.g., E2/3, E4, I4) or phase duration (e.g., I8, I1, E1)
