PUB - Publikationen an der Universität Bielefeld

Walking stick insects, Carausius morosus , explore their environment ahead through coordinated, rhythmic movements of their antennae. Antennal contacts during forward walking cause fast adaptive movements of the front legs towards the detected object whereas mechanical tension applied to the antennae induces backward walking. Thus, different tactile information has to be processed and conveyed to the neural network of the prothoracic step pattern generator. In the present study we examined the effects of antennal stimulation on neurons of the leg motor system in restrained animals. The responses of femoro-tibial (FT), coxo-trochanteral (CT) and subcoxal motoneurons as well as the nonspiking premotor interneuron E4 to manually applied tension to the antennae and tactile stimulation of the flagellum by means of a paint brush were recorded. The mechanical tension was applied by pinching the ipsi- or contralateral antenna between two fingers. This stimulation leads to a clear-cut response in the membrane potential of all tested motoneurons and the premotor interneuron E4. Most of the examined neurons were depolarized. Sustained pinching with fingers resulted in sustained tonic depolarizations. In contrast, the same stimulus carried out with gloves or by an elastic live insect forceps induces no reaction in the membrane potentials of the FT- and CT-motoneurons. Pure tactile stimulation to the ipsi- or contralateral flagellum, carried out by a paint brush, also elicits no response in these motoneurons. In contrast, subcoxal motoneuron pools react to both stimulations with gloves and a live insect forceps as well as to the paint brush stimulus. However, most often only slight depolarizations were observed. This indicates that the subcoxal motoneurons are more sensitive to pure tactile stimulation than the motoneurons of the distal joints. We conclude that pinching with fingers is a combination of tactile and contact-chemosensory stimulation that excites all leg motoneuron pools and the premotor interneuron E4. The motoneurons of antagonistic muscles show co-contractions to the stimuli. A pure chemosensory stimulation of the antennae with different scents (essential oils) shows no effects even not in the membrane potential of the subcoxal motoneurons which are sensitive to slight mechanical stimulation. Thus, a strong contact-chemosensory component might be involved in initiating backward walking. In the active, walking animal tactile information has to be processed in all prothoracic motoneurons, for example to avoid obstacles, as can be deduced from behavioral experiments. Therefore, we assume an activity dependent gating mechanism.