Synaptic vesicle fusion along axons is driven by myelination and subsequently accelerates sheath growth in an activity-regulated manner

To study activity-regulated myelination, we imaged synaptic vesicle fusion along single axons in living zebrafish, and found, surprisingly, that axonal synaptic vesicle fusion is driven by myelination. This myelin-induced axonal vesicle fusion was enriched along the unmyelinated domains into which newly-formed sheaths grew, and was promoted by neuronal activity, which in turn accelerated sheath growth. Our results indicate that neuronal activity consolidates sheath growth along axons already selected for myelination.