An Imbalancing Act: Gap Junctions Reduce the Backward Motor Circuit Activity to Bias C. elegans for Forward Locomotion

The main role of AVB is to influence B-class motorneurons to turn on and off forward locomotion, whereas the main role of AVA is to influence A-class motorneurons to turn on and off backward locomotion.

“the main role of the AVB class is to influence the B-class motorneuron to turn on and off forward locomotion, whereas the main role of AVA is to influence the A-class motorneuron to turn on and off backward locomotion (Kawano et al., 2011; Qi, Garren, Shu, Tsien, & Jin, 2012; Rakowski, Srinivasan, Sternberg, & Karbowski, 2013; Zheng, Brockie, Mellem, Madsen, & Maricq, 1999).”

The C. elegans locomotion pattern is characterized by A- and B-class motorneurons oscillating.

“(1) Oscillation criterion: the A- and B-class motorneurons oscillate (Faumont et al., 2011; Haspel et al., 2010; Kawano et al., 2011);”

During forward locomotion, AVB activity is active while AVA is suppressed; during backward locomotion, AVB is suppressed while AVA is active.

“During forward locomotion, AVB activity is active, while AVA is suppressed. During backward locomotion, the opposite is true: AVB is suppressed, while AVA is active.”

The C. elegans locomotion pattern is characterized by B-class activity exceeding A-class activity during forward locomotion, whereas A-class activity exceeds B-class activity during backward locomotion.

“(3) dominance criterion: B-class activity exceeds A-class activity during forward locomotion, whereas A-class activity exceeds B-class activity during backward locomotion (Haspel et al., 2010; Kawano et al., 2011).”

The C. elegans locomotion pattern is characterized by dorsal and ventral oscillations in each class of cells occurring in antiphase.

“(2) phase criterion: the dorsal and ventral oscillations in each class of cells occur in antiphase in order to drive the alternating dorsoventral bends responsible for locomotion (Faumont et al., 2011; Kawano et al., 2011);”

During backward locomotion in the full circuit, the minimum activity of VA remains quite high, as in the worm.

“As in the worm (Haspel et al., 2010; Kawano et al., 2011), during backward locomotion in the full circuit the minimum activity of VA remains quite high.”

When the model circuit is driven by realistic AVA/AVB input taken from calcium imaging of freely moving worms, it produces realistic-looking motor output.

“when the model circuit is driven by realistic AVA/AVB input taken from calcium imagining of freely moving worms (see Figure 1F in Kawano et al., 2011; reproduced in Figure 5A), it produces realistic-looking motor output”

The fitness function's three components capture the main three features of the in vivo calcium recordings for these motorneurons during forward and backward locomotion.

“For each locomotion direction, neural traces were evaluated using three components that capture the main three features of the in vivo calcium recordings for these motorneurons during forward and backward locomotion (Faumont et al., 2011; Kawano et al., 2011).”

Oscillations in the limited range (A=0.3) match what has been observed experimentally.

“Oscillations in this limited range match what has been observed experimentally (Kawano et al., 2011), which ultimately allows for one class of motorneurons to show dominance over the other one by oscillating in the top third or the bottom third of the full output range.”