Neuronal mechanisms underlying flexible locomotor behaviour development

Brain development during infancy is crucial in gaining the full complexity of adult behaviour. During this period, the number of new neuronal connections increases dramatically. Recent research shows that such new connections are segregated based on their ontogeny, forming new circuit modules in parallel to existing ones. Yet, how the brain orchestrates such parallel modules to produce flexible behaviour remains elusive. To reveal the mechanism at play, we focus on the identifiable vsx2 expressing reticulospinal neurons in larval zebrafish that form two distinct circuit modules involved in locomotion: an early- developing module for fast, whole-body movements and a late-developing module for slower, tail- restricted movements. We find how these neurons are recruited in a specific sequence and begin to reveal how this sequence is implemented at the circuit level. Our study sheds light on the specific circuit dynamics underlying flexible locomotor sequences and paves the way for elucidating the circuit mechanism for coordinating early- and late-developing circuit modules.