In the central nervous system, cells called oligodendrocytes create myelin, the electrical insulation that sheathes the axons of neurons. The formation of myelin dramatically increases the speed of nerve signals and is critical to nervous system development and physiology, and demyelination can play a central role in disease (1–4). Many of the mechanisms underlying myelination, however, remain unclear. On page 1647 of this issue, Wake et al. (5) expand our understanding. They demonstrate that electrically active axons can induce myelin formation by vesicular release of glutamate that signals nearby oligodendrocytes to start local production of a myelin-building protein. The finding provides novel insights into how experience can influence brain development and physiology.