The role of sensory influences in the control of motor activity of a stepping insect leg

For a long time the focus of the discourse in motor control research was on stereotypical movements, such as forward walking. My thesis emphasizes how different levels of neuronal motor control contribute to the processing of task-dependent locomotor behavior. This resulted in three main questions. First, how sensory feedback affects the timing and magnitude of muscle activity in general. Second, how this feedback is processed in changes of task-dependent movement behavior. And third, what mechanisms are used in the neural network to evoke an adaptive capability. Central pattern generators (CPGs) generate a rhythmic, alternating motor activity that is in turn modulated by sensory feedback in timing and in magnitude. In the middle leg of the stick insect Carausius morosus, two major groups of sense organs measure either the load or the movement and positional parameters of the leg. The role of these sense organs was examined in my work. In the first study, the influence of campaniform sensilla (CS) on magnitude and the timing of stance phase muscles was examined. For this purpose, a trapdoor setup with a slippery surface was used. The animal either stepped on a slippery surface with ground contact or they stepped into a hole when the trapdoor was lowered (SIH). Through ground contact, the legs are loaded and thereby activate the leg CS. During SIH this sensory feedback is missing. Through ablation experiments, I was able to show, in addition to CS, an additional sense organ that activated the flexor tibiae (FlxTi) through their sensory information of the touchdown (TD). In all stance phase muscles, except for the depressor trochanteris (DepTr), the strength of muscle activity increased through the TD. In the second study, the control on the timing of activation, especially of the extensor tibiae (ExtTi) muscle by sensory feedback of the femoral chordotonal organ (fCO) was investigated. Here, a focus was set on the processing of this feedback to movement changes, in particular curve walking. The fCO measures various parameters of the knee joint movement (femorotibial [FT] joint) and position. Feedback from the fCO generates resistance reflexes (RR), which are used to maintain the posture. Assistance reflex or reflex reversals (AR), which assist stance phase activity in the active animal, were also observed. During curve walking, the legs on each side of the animal have different kinematics. The leg inside of a curve (inside) is mainly moved around the FT joint, while the leg outside of the curve (outside) is mostly moved in the hip joint (thoracocoxal [ThC] joint). ExtTi motor neurons (MN) were activated during an AR at a certain angular difference. This was independent of the function as inside or outside leg. Further, the fCO stimulation induced ARs more often in the inside leg while RRs occurred more frequently in the outside leg. In addition, more ARs were generated through fCO stimulation at slower velocities and larger starting angles. The protractor coxae (ProCx) showed increased activation through fCO stimulation in the outside leg, while the levator coxae (LevTr) showed no difference in the reaction between the two leg xii functions. Thus, it could be shown in this part of my thesis that feedback from the fCO can cause task-specific motor activity. In the third study, the mechanisms that might be responsible for the decrease in the response to fCO stimulation when the leg is used as the outside leg were investigated. The monosynaptic connection of fCO afferents to ExtTi MNs, as well as to nonspiking interneurons (NSIs), was reduced. Similar observations were made in ExtTi MNs throughout the complete fCO stimulation. Furthermore, ExtTi MNs received greater tonic depolarization and their membrane input resistance was reduced. In my dissertation I could show that motor activity is influenced by various sense organs. These influences can be adapted to changes in movement, whereby I could reveal one mechanism leading to task-dependent motor activity. The influence of NSIs or presynaptic inhibition on changes in task dependent motor output remains unclear.