We used our biophysical measurements to calculate the effect of proprioception on undulatory waves in surroundings with different viscosities and uncovered a compelling explanation for the adaptation of undulatory wavelength on external load. At low loads, the worm undulates with a long wavelength. At high loads, the worm undulates with a short wavelength. This dependence has an intuitive biomechanical explanation. As external viscosity increases, it takes longer for a posterior body region to bend in response to any curvature change in its anterior neighbor. Increasing the time scale of the

bending response increases the phase difference between the shapes of neighboring body segments, leading to a smaller undulation wavelength. The small size and experimental accessibility of the C. elegans motor circuit allows the possibility of modeling locomotion that integrates the dynamics of all neuronal and muscular components. Our OTX015 clinical trial results suggest Luminespib in vivo that a full model of C. elegans locomotion must integrate the biomechanics of undulatory movement with neuromuscular activity to properly incorporate the role of proprioception within the motor circuit. Wild-type, transgenic, and mutant worms were cultivated using standard methods (Brenner, 1974). Detailed strain information can be found in the Supplemental Information. The transgenic worms used in all optogenetic

experiments were cultivated in the dark at 20°C on NGM plates with Escherichia coli OP50 and all-trans retinal. We performed all experiments using adult hermaphrodites within a few hours after their final molt. Custom microfluidic devices were fabricated in PDMS using soft lithography techniques. In the pneumatic microfluidic device, the channel was flanked by two chambers that could be alternatively enough pressurized and depressurized with a valve system under computer control using custom software written in LabVIEW (National Instruments, Austin, TX). We loaded each microfluidic channel with NGM buffer

or dextran solution (∼20% dextran in NGM [wt/vol] in most cases). An individual worm was flowed into the inlet of each microfluidic channel and worm position within each channel was manually controlled by syringes connected to polyethylene tubing. Experiments were performed on Nikon microscopes (TE2000 or Eclipse LV150) under 4× magnification with dark-field illumination. Image sequences were taken by a CCD camera (Imaging Source) and recorded on a computer at 30 Hz using IC Capture software (Imaging Source). Image analysis was performed using custom software written in MATLAB (MathWorks, Inc. Natick, MA) following methods described in (Fang-Yen et al., 2010). We imaged calcium dynamics within muscle cells of worms partially trapped in microfluidic channels, using methods similar to those described in (Chen, 2007). GCaMP3 and RFP were excited by LEDs filtered at 448–492 nm and 554–572 nm, respectively, using Semrock single-bandpass filters.