Ciliomotor circuitry underlying whole-body coordination of ciliary activity in the Platynereis larva

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Summary:

This well-written and exciting paper describes a partial connectome for the larva of the annelid Platynereis. Locomotion in ciliated animals has long been interesting from a biomechanics point of view, but whole-body coordination of ciliary movements requires internal circuitry that has long been mysterious. This paper reconstructs this motor circuitry through reconstruction of serial electron micrographs, identifying all multiciliated cells as well as all the ciliomotor neurons presynaptic to them. Such circuitry has long been studied in more conventional motor systems, and understanding how such circuitry might be adapted to such a different swimmer is of fundamental interest and importance. The authors then characterize the neurochemistry of these neurons, identifying cholinergic, serotonergic and catecholaminergic/peptidergic cells. Finally, using whole-animal calcium imaging, the authors functionally characterize the motor circuitry by examining temporal correlations between neurons and their synaptic targets. Basic puzzles like the locus of the rhythm generator remain to be discovered, but the authors are very clear about outstanding questions in the field.

Minor points:

Textual clarification: in general, the paper seems to assume that the identified neurotransmitters/modulators act at synapses, but amines and peptides in particular are known to often act extrasynaptically and at a distance between neurons that may or may not be synaptically connected. The authors should discuss this issue in the text.