Functional Analysis of the Motor Circuit of Juvenile Caenorhabditis elegans

Caenorhabditis elegans generates alternating dorsal-ventral bending waves across developmental stages. The overall level of dorsal and ventral muscle output is symmetric. In adults, this behavioral symmetry correlates with anatomy symmetry: distinct pools of cholinergic motor neurons activate dorsal and ventral body wall muscles, whereas distinct pools of GABAergic motor neurons contra-laterally inhibit dorsal and ventral muscles, respectively. However, in the first stage larva (L1), cholinergic motor neurons only make neuromuscular junctions to the dorsal muscle, and GABAergic motor neurons to the ventral muscle. How does an asymmetric motor circuit produce symmetric motor output? In the past five years, my colleagues and I undertook anatomical and functional analyses of the L1 larva motor circuit to address this question. First, my colleagues fully reconstructed the connectivity of a complete L1 larva, from which I identified all candidate cellular components for muscle activity. Next, to elucidate the functional contribution of these candidates, I developed an all-optical manipulation method that allows fast and systemic interrogation of functional connections of neural circuits. Lastly, I led the effort to combine free-moving calcium imaging, cellular ablation, and all-optical interrogation to pinpoint mechanisms for the symmetric L1 muscle output. Collectively, results from these studies show that all cells that make anatomic synaptic connections are insufficient to explain the symmetry of body wall muscle excitability in L1 C. elegans. Instead, the anatomically asymmetric L1 motor circuit is functionally compensated by extra-synaptic transmissions from both excitatory premotor interneurons and inhibitory motor neurons to operate as a symmetric circuit. Excitatory premotor interneurons can bypass the motor neurons layer to directly regulate excitability of muscle cells, thus assuming dual roles. Animals with distinct wirings can produce similar motor patterns. Our findings in the juvenile C. elegans motor circuit show that structurally divergent neural circuits can functionally converge on the functional output, and that neurons can multiplex. This study further demonstrates a larger role of functional communication devoid of ultrastructural features in a developing neural circuit.