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See Inside August/September 2007

White Matter Listens In

Brain cells thought to be bystanders communicate using chemicals

Our brain cells are chattier than previously thought, according to a new study. Cells in white matter, once believed to passively relay information between neurons, were found to eavesdrop on the messages they carried and to receive chemical signals from other cells.

Until now, researchers believed our brain had a basic division of labor between gray matter and white matter. In gray matter, neurons form dense networks and process information by sending signals along fingerlike projections called axons. White matter was thought to play a supporting role by producing a white protein called myelin that coats the axons and allows them to send signals more quickly.

But a new mouse study led by neurobiologist Dwight Bergles of Johns Hopkins University found that certain white matter cells form synapses to listen in on the signals in axons. Just as neurons in gray matter connect to and communicate with one another via axonal projections, oligodendrocyte precursor cells in white matter connect to the axons running past them to pick up the chemical signals those axons release.

“There's the same kind of signaling in white matter as in gray matter,” Bergles says. This surprising finding may explain why previous studies suggest that learning new skills increases the amount of myelin in specific regions of white matter. Scientists have long known that when we learn, our brain forms new synaptic connections among gray matter neurons to maximize information transfer—much like building new lanes on an increasingly busy highway. White matter could respond similarly, ramping up myelin production and raising the speed limit along certain routes. Although it is not yet known exactly how this myelin buildup takes place, these results suggest that white matter could be picking up “go” or “stop” signals from axons, directing the myelination process and setting the optimal speed for information transfer in our brain.

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