Summary Locomotion requires coordinated motor activity throughout an animal’s body. In both vertebrates and invertebrates, chains of coupled Central Pattern Generators (CPGs) are commonly evoked to explain local rhythmic behaviors. In C. elegans, we report that proprioception within the motor circuit is responsible for propagating and coordinating rhythmic undulatory waves from head to tail during forward movement. Proprioceptive coupling between adjacent body regions transduces rhythmic movement initiated near the head into bending waves driven along the body by a chain of reflexes. Using optogenetics and calcium imaging to manipulate and monitor motor circuit activity of moving C. elegans held in microfluidic devices, we found that the B-type cholinergic motor neurons transduce the proprioceptive signal. In C. elegans, a sensorimotor feedback loop operating within a specific type of motor neuron both drives and organizes body movement.

To navigate different environments, an animal must be able to adapt its locomotory gait to its physical surroundings. The nematode Caenorhabditis elegans, between swimming in water and crawling on surfaces, adapts its locomotory gait to surroundings that impose approximately 10,000-fold differences in mechanical resistance. Here we investigate this feat by studying the undulatory movements of C. elegans in Newtonian fluids spanning nearly five orders of magnitude in viscosity. In these fluids, the worm undulatory gait varies continuously with changes in external load: As load increases, both wavelength and frequency of undulation decrease. We also quantify the internal viscoelastic properties of the worm's body and their role in locomotory dynamics. We incorporate muscle activity, internal load, and external load into a biomechanical model of locomotion and show that (i) muscle power is nearly constant across changes in locomotory gait, and (ii) the onset of gait adaptation occurs as external load becomes comparable to internal load. During the swimming gait, which is evoked by small external loads, muscle power is primarily devoted to bending the worm's elastic body. During the crawling gait, evoked by large external loads, comparable muscle power is used to drive the external load and the elastic body. Our results suggest that C. elegans locomotory gait continuously adapts to external mechanical load in order to maintain propulsive thrust.H ow do neural circuits produce and regulate the rhythmic patterns of muscle activity that drive animal locomotion? The nematode Caenorhabditis elegans-with its well-mapped nervous system (1), relatively simple anatomy (2), and rhythmic undulatory movements (3)-is a promising model for exploring the neural basis of locomotion. To fully understand motor behavior we also need to understand C. elegans locomotory biomechanics: how muscle activity produces movement within the mechanical framework of the worm's body and its physical environment.C. elegans moves forward by propagating undulatory waves in a dorsal-ventral plane from head to tail (3). Bending is generated by alternating contraction and relaxation of two dorsal and two ventral muscle groups running along the length of the worm's body (2). Both the shape and speed of these undulations change in response to the physical environment (3, 4). When moving on moist surfaces such as agarose gels, C. elegans exhibits a crawling gait characterized by undulations with low frequency and short wavelength (5). By contrast, when moving through water, C. elegans exhibits a swimming gait characterized by undulations with higher frequency and longer wavelength ( Table 1). The differences in the size and speed of undulations are modest in comparison with the difference in the scales of physical force during swimming and crawling. At the size and speed of C. elegans, forces due to surface tension (surface tension holds the crawling animal to the agar surface) are approximately 10,000-fold larger than forces due to viscosity when swimming ...

The internal brain dynamics that link sensation and action are arguably better studied during natural animal behaviors. Here, we report on a novel volume imaging and 3D tracking technique that monitors whole brain neural activity in freely swimming larval zebrafish (Danio rerio). We demonstrated the capability of our system through functional imaging of neural activity during visually evoked and prey capture behaviors in larval zebrafish.