Distributed control of motor circuits for backward walking in Drosophila.

How do descending inputs from the brain control leg motor circuits to change how an animal walks? Conceptually, descending neurons are thought to function either as command-type neurons, in which a single type of descending neuron exerts a high-level control to elicit a coordinated change in motor output, or through a population coding mechanism, whereby a group of neurons, each with local effects, act in combination to elicit a global motor response. The Drosophila Moonwalker Descending Neurons (MDNs), which alter leg motor circuit dynamics so that the fly walks backwards, exemplify the command-type mechanism. Here, we identify several dozen MDN target neurons within the leg motor circuits, and show that two of them mediate distinct and highly-specific changes in leg muscle activity during backward walking: LBL40 neurons provide the hindleg power stroke during stance phase; LUL130 neurons lift the legs at the end of stance to initiate swing. Through these two effector neurons, MDN directly controls both the stance and swing phases of the backward stepping cycle. These findings suggest that command-type descending neurons can also operate through the distributed control of local motor circuits.

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.

Inter-leg coordination in the control of walking speed in Drosophila.

Legged locomotion is the most common behavior of terrestrial animals and it is assumed to have become highly optimized during evolution. Quadrupeds, for instance, use distinct gaits that are optimal with regard to metabolic cost and have characteristic kinematic features and patterns of inter-leg coordination. In insects, the situation is not as clear. In general, insects are able to alter inter-leg coordination systematically with locomotion speed, producing a continuum of movement patterns. This notion, however, is based on the study of several insect species, which differ greatly in size and mass. Each of these species tends to walk at a rather narrow range of speeds. We have addressed these issues by examining four strains of Drosophila, which are similar in size and mass, but tend to walk at different speed ranges. Our data suggest that Drosophila controls its walking speed almost exclusively via step frequency. At high walking speeds, we invariably found tripod coordination patterns, the quality of which increased with speed as indicated by a simple measure of tripod coordination strength (TCS). At low speeds, we also observed tetrapod coordination and wave gait-like walking patterns. These findings not only suggest a systematic speed dependence of inter-leg movement patterns but also imply that inter-leg coordination is flexible. This was further supported by amputation experiments in which we examined walking behavior in animals after the removal of a hindleg. These animals show immediate adaptations in body posture, leg kinematics and inter-leg coordination, thereby maintaining their ability to walk.

Tethered stick insect walking: a modified slippery surface setup with optomotor stimulation and electrical monitoring of tarsal contact.

A modified and improved setup based on Epstein and Graham [Epstein S, Graham D. Behaviour and motor output of stick insects walking on a slippery surface. I. Forward walking. J Exp Biol 1983;105: 215-29] to study straight and curve walking in the stick insect was developed and applications for its use are described. The animal is fixed on a balsa stick and walks freely on a slippery surface created with a thin film of a glycerin/water solution on a black, Ni-coated, polished brass plate. The glycerine/water ratio controls the viscosity of the lubricant and thereby the forces necessary to move the legs of the stick insect. A small amount of NaCl is added to ensure electric conductivity. Walking is induced through an optomotor stimulus given by two stripe-projectors producing rotatory and translatory stimuli to influence walking direction. The walking pattern is monitored in two ways: (1) tarsal contact with the slippery surface is measured electrically using a lock-in-amplifier. The tarsal contact signal allows correlation with the activity in different muscles of the stick insect leg recorded with EMG electrodes; (2) leg kinematics in the horizontal plane is monitored using synchronized high speed video. This setup allows us to determine the coupling of activity in different leg muscles to either swing or stance phase during straight and curve walking in the intact animal or the reduced single-leg preparation with a high time resolution.