Thorax-Segment- and Leg-Segment-Specific Motor Control for Adaptive Behavior.

We have just started to understand the mechanisms underlying flexibility of motor programs among segmental neural networks that control each individual leg during walking in vertebrates and invertebrates. Here, we investigated the mechanisms underlying curve walking in the stick insect Carausius morosus during optomotor-induced turning. We wanted to know, whether the previously reported body-side specific changes in a two-front leg turning animal are also observed in the other thoracic leg segments. The motor activity of the three major leg joints showed three types of responses: 1) a context-dependent increase or decrease in motor neuron (MN) activity of the antagonistic MN pools of the thorax-coxa (ThC)-joint during inside and outside turns; 2) an activation of 1 MN pool with simultaneous cessation of the other, independent of the turning direction in the coxa-trochanteral (CTr)-joint; 3) a modification in the activity of both FTi-joint MN pools which depended on the turning direction in one, but not in the other thorax segment. By pharmacological activation of the meso- or metathoracic central pattern generating networks (CPG), we show that turning-related modifications in motor output involve changes to local CPG activity. The rhythmic activity in the MN pools of the ThC and CTr-joints was modified similarly to what was observed under control conditions in saline. Our results indicate that changes in meso- and metathoracic motor activity during curve walking are leg-joint- and thorax-segment-specific, can depend on the turning direction, and are mediated through changes in local CPG activity.

Body side-specific changes in sensorimotor processing of movement feedback in a walking insect.

Feedback from load and movement sensors can modify timing and magnitude of the motor output in the stepping stick insect. One source of feedback is stretch reception by the femoral chordotonal organ (fCO), which encodes such parameters as the femorotibial (FTi) joint angle, the angular velocity, and its acceleration. Stimulation of the fCO causes a postural resistance reflex, during quiescence, and can elicit the opposite, so-called active reaction (AR), which assists ongoing flexion during active movements. In the present study, we investigated the role of fCO feedback for the difference in likelihood of generating ARs on the inside vs. the outside during curve stepping. We analyzed the effects of fCO stimulation on the motor output to the FTi and the neighboring coxa-trochanter and thorax-coxa joints of the middle leg. In inside and outside turns, the probability for ARs increases with increasing starting angle and decreasing stimulus velocity; furthermore, it is independent of the total angular excursion. However, the transition between stance and swing motor activity always occurs after a specific angular excursion, independent of the turning direction. Feedback from the fCO also has an excitatory influence on levator trochanteris motoneurons (MNs) during inside and outside turns, whereas the same feedback affects protractor coxae MNs only during outside steps. Our results suggest joint- and body side-dependent processing of fCO feedback. A shift in gain may be responsible for different AR probabilities between inside and outside turning, whereas the general control mechanism for ARs is unchanged.NEW & NOTEWORTHY We show that parameters of movement feedback from the tibia in an insect during curve walking are processed in a body side-specific manner, and how. From our results it is highly conceivable that the difference in motor response to the feedback supports the body side-specific leg kinematics during turning. Future studies will need to determine the source for the inputs that determine the local changes in sensory-motor processing.

The role of leg touchdown for the control of locomotor activity in the walking stick insect.

Much is known on how select sensory feedback contributes to the activation of different motoneuron pools in the locomotor control system of stick insects. However, even though activation of the stance phase muscles depressor trochanteris, retractor unguis, flexor tibiae and retractor coxae is correlated with the touchdown of the leg, the potential sensory basis of this correlation or its connection to burst intensity remains unknown. In our experiments, we are using a trap door setup to investigate how ground contact contributes to stance phase muscle activation and burst intensity in different stick insect species, and which afferent input is involved in the respective changes. While the magnitude of activation is changed in all of the above stance phase muscles, only the timing of the flexor tibiae muscle is changed if the animal unexpectedly steps into a hole. Individual and combined ablation of different force sensors on the leg demonstrated influence from femoral campaniform sensilla on flexor muscle timing, causing a significant increase in the latencies during control and air steps. Our results show that specific load feedback signals determine the timing of flexor tibiae activation at the swing-to-stance transition in stepping stick insects, but that additional feedback may also be involved in flexor muscle activation during stick insect locomotion. With respect to timing, all other investigated stance phase muscles appear to be under sensory control other than that elicited through touchdown.

A laser-supported lowerable surface setup to study the role of ground contact during stepping.

We introduce a laser-supported setup to study the influence of afferent input on muscle activation during walking, using a movable ground platform. This approach allows investigating if and how the activity of stance phase muscles of an insect (e.g. stick insect) responds to a missing ground contact signal. The walking surface consists of a fixed and a lowerable part, which can be lowered to defined levels below the previous ground level at any time point during a walking sequence. As a consequence, the leg under investigation finds either a lower ground level or no ground support at all. The lowerable walking surface consists of a 49 mm × 34 mm stainless steel surface, made slippery and equipped for tarsal contact monitoring, similar to the system that was described by Gruhn and colleagues (Gruhn et al., 2006). The setup controller allows pneumatic lowering of the surface and subsequent detection of tarsal entry into the previous ground level with the help of a thin sheet of laser light and a corresponding detector. Here, we describe basic properties of the new setup and show the results of first experiments to demonstrate its use for the study of sensory and central influences in stepping of a small animal. In the experiments, we compare the effect of ground-support ("control") with either steps into the hole (SiH), ground support at a lower surface level, or the amputation of the tarsus on the onset of EMG activity in the flexor tibiae muscle of the stick insect.

Calcium buffering and clearance in spider mechanosensory neurons.

Spider VS-3 mechanoreceptor neurons have a low-voltage-activated Ca2+ current that raises intracellular calcium concentration [Ca2+] when they are depolarized by agonists of GABAA receptors or fire action potentials. The Ca2+ rise produces negative feedback by modulating the mechanoreceptor current and regulates Ca2+- and voltage-activated K+ currents. However, nothing is known about Ca2+ buffering in VS-3 neurons. Dynamic changes in VS-3 neuron intracellular [Ca2+] were measured using the fluorescent Ca2+ indicator Oregon Green BAPTA-1 (OG488) to understand Ca2+ buffering and clearance. Intracellular OG488 concentration increased slowly over more than 2 h as it diffused through a sharp intracellular microelectrode and spread through the cell. This slow increase was used to measure endogenous Ca2+ buffering and clearance by the added buffer technique, with OG488 acting as both added exogenous buffer and Ca2+ indicator. [Ca2+] was raised for brief periods by regular action potential firing, produced by pulsed electric current injection through the microelectrode. The resulting rise and fall of [Ca2+] were well fitted by the single compartment model of Ca2+ dynamics. With earlier ratiometric [Ca2+] estimates, these data gave an endogenous Ca2+ binding ratio of 684. Strong Ca2+ buffering may assist these neurons to deal with rapid changes in mechanical inputs.