To assess whether the delayed bending of the posterior region represented mechanical damping by the external viscous fluid or internal delays within the neuromuscular network, we studied worms in fluids of different viscosity (Figures 5D–5F). We found that the bending delay was roughly constant, GSK2656157 mw ∼300 ms, in fluids ranging from 1 mPa·s (the viscosity of water) to ∼100 mPa·s. In more viscous fluids, the bending delay began to increase,
becoming ∼1 s at 300 mPa·s. These results suggest that ∼300 ms represents an upper bound for delays within the neuromuscular network, which are rate-limiting at low viscosities. These neuromuscular delays might reflect delays in synaptic transmission and/or the limiting speed of muscle contraction. The C. elegans wiring selleck diagram offers a small number of candidate cell types within the motor circuit that might play roles in generating or propagating a local proprioceptive signal: the A-type cholinergic motor
neurons, B-type cholinergic motor neurons, the D-type GABAergic motor neurons, and muscle cells. One neuron outside the core motor circuit, the DVA interneuron, has also been shown to exhibit proprioceptive properties ( Li et al., 2006). We sought to determine which cell type was responsible for coupling the bending activities of adjacent body regions through proprioception. First, we trapped transgenic worms that expressed halorhodopsin in all cholinergic motor neurons (Punc-17::NpHR) in the pneumatic devices and illuminated them with green light. We found that light-induced hyperpolarization of the cholinergic neurons prevented the posterior body regions from following induced changes in the curvature of the anterior region ( Figures 6A–6C and Movie S8). Instead, optogenetic inactivation of the cholinergic neurons locked the posterior region in the posture as it was immediately preceding illumination. Second, we studied vab-7 mutants, which have specific defects PDK4 in the morphology of the dorsal B-type
cholinergic motor neurons. In these mutants, the DB neurons reverse the orientation of their axons so that they project anteriorly instead of posteriorly ( Esmaeili et al., 2002) ( Figure S3A) The vab-7 mutation does not affect the ventral B-type motor neurons. During unrestrained forward movement, the bending wave near the head of vab-7 mutants was normal. However, the bending wave that propagates to posterior regions was biased toward the ventral side ( Figures S3B and S3D). When we trapped vab-7 mutants in the pneumatic channels, the posterior region was only able to follow channel bending to the ventral side, not to the dorsal side ( Figures S3C, S3F, and S3G). These results suggest that the dorsal and ventral B-type cholinergic motor neurons are each responsible for propagating dorsal and ventral curvatures to posterior body regions.