Neurons have always been the stars of brain research, but scientists are now realizing that nonneuronal cells known as glia—which make up around 90 percent of cells in the brain—are not the mild-mannered understudies they appeared to be. Some glia may even fire electrical signals, a finding that overturns a central dogma of neuroscience that holds that neurons are the only cells in the brain with such signaling ability.
Last winter, when neuroscientists at University College London examined glia known as oligodendrocyte precursor cells (OPCs), they were astounded to find that, just like neurons, one subtype fired electrical signals in response to electrical stimulation. Before this study little was known about the function of OPCs, says study leader Ragnhildur Karadottir, except that they could develop into new oligodendrocytes, a type of glial cell that forms an insulating sheath around neurons like the rubber on an electrical cord.
“We were very surprised,” Karadottir says. “The first thing one learns in neuroscience is that neurons fire action potentials and glia do not.” The researchers suspect that, in these glia, action potentials—the rapid electric currents that travel along nerves—might serve as a signal to insulate an active neuron.
Other recent findings are further eroding the idea that glia merely provide food and support to neurons. Scientists have known for years that glial cells play an integral role at the neuromuscular junction, where nerves meet muscle in the body. But glia in the brain are much more difficult to study because they are harder to isolate, image and grow in the lab.
So a number of scientists are focusing on alternative ways to study glia. This winter a group of researchers at Stanford University compared the active genes of neurons, oligodendrocytes and astrocytes, the star-shaped glial cells that fill the spaces between neurons. Some of the astrocyte genes they found are important for phagocytosis, a crucial biological process by which healthy cells engulf and destroy bacteria or dying cells. This genetic footprint may indicate that astrocytes help to keep the brain clean of dying cells and scar tissue, says John Cahoy, the graduate student who led the study.
Many of the newly identified genes are completely unknown, however, and Cahoy says that his research on glia is just a start. As neuroscientists refine their understanding of the brain, they are realizing that cognition is even more complex than anybody imagined. “We’re just opening the door on understanding how glial cells interact with neurons,” Cahoy says. (For more on the role of glia in the brain, see “The Forgotten Brain Emerges,” by Claudia Krebs, Kerstin Httmann and Christian Steinhuser; Scientific American Mind, December 2004.)
This story was originally printed with the title, "The Other Brain Cells".