Some 90 percent of the cells in your noggin are not neurons but glia¿brain cells that until now were credited with the organ's most unglamorous work. Indeed, scientists guessed that glia served only as scaffolding for the more important neurons or that perhaps they collected garbage molecules from synapses, the channels over which neurons communicate. A paper in the January 26th issue of Science, however, reveals that glial cells called astrocytes actually control the number of synapses neurons can form with each other. So, far from playing a supporting role, glia may in fact actively participate in learning and memory.

Ben Barres and his colleagues at Stanford University expected more from glia starting in 1997, when they discovered that neurons grown with astrocytes in culture are 10 times more active than neurons grown alone. What they didn't know was whether the increased activity resulted from more synapses or increased efficiency at the synapses. This time they performed a number of tests on neurons called retinal ganglion cells, finding that they, too, were seven times more responsive to an array of stimuli when they were grown near glia. And by way of staining and imaging, they were also able to conclude that glia-exposed retinal ganglion cells featured seven times as many synapses.

"In summary, these results show that the total number of synapses on a neuron is not an intrinsic property of that neuron, but can be profoundly regulated by extrinsic signals," the authors write. "In the absence of glia, neurons in culture have only a limited ability to form synapses. Astrocytes greatly increase the number of structurally mature, functional synapses and are necessary to maintain synaptic stability."