The Quiet Cells

New research is unlocking the secret role of glia, the brain cells that were long considered more structural than functional. As it turns out, though, glia may be even more responsive to certain types of stimuli than neurons are.

An astrocyte strikes a pose for the camera.

One type of glial cells – known as astrocytes because of their star-like shape – compose far more of the brain’s cell population than neurons do. Because glia didn’t seem to be synapsing with neurons, most scientists had assumed their roles were to hold neuronal structures together, and to help shape neurons’ structural development – in fact, the word “glia” itself comes from the Greek word for glue.

“Electrically, astrocytes are pretty silent,” [said MIT neuroscientist] James Schummers. “A lot of what we know about neurons is from sticking electrodes in them. We couldn’t record from astrocytes, so we ignored them.”

But a 2008 MIT study called that idea into question, by showing that glia help regulate cerebral blood flow. More recently, studies led by Dr. Alexander Gourine at the University of Bristol and Dr. James Schummers at the Max Planck Florida Institute have killed off the old notion of passive glia for good.

Gourine’s and Schummers’ research has proven that astrocytes respond to slight decreases in blood pH – which reflect a rising level of carbon dioxide – by releasing calcium ions (Ca2+) and adenosine triphosphate (ATP), both of which play crucial parts in neurons’ synaptic signaling:

ATP propagated astrocytic Ca2+ excitation, activated chemoreceptor neurons, and induced adaptive increases in breathing … This demonstrates a potentially crucial role for brain glial cells in mediating a fundamental physiological reflex.

In other words, astrocytes directly signal neurons to let us know when and how much we need to breathe – that’s about as fundamental as reflexes get.

And researchers suspect astrocytes may be keeping other secrets too. Some think they may play a role in memory formation, while others think they may play a more general role in regulating the levels of neurotransmitters that hang out around synapses, and in determining where the brain’s blood supply should be focused.

This could be the beginning of a major paradigm shift, because many of today’s functional studies of the brain use fMRI scans, which measure changes in blood flow to different brain areas. If astrocytes are actively involved in controlling this process, fMRI may not be telling us exactly what we thought:

Questions have plagued [fMRI] studies, as it is difficult to know what is happening when a particular part of the brain “lights up” in MRI images. [One fMRI researcher] says that it’s important for scientists to be aware that MRI images reflect the status of astrocytes, and that “things that influence astrocytes will influence the signal.”

There’s an even more positive side to these developments, too: understanding the roles of glia may help neuroscientists gain a much clearer understanding of baffling disorders like autism and schizophrenia, because genes linked to these problems seem to be commonly expressed in astrocytes. All in all, we seem to watching the birth of a fascinating new field of neuroscience research.

In the meantime, you might take a second to pause, breathe, and thank your astrocytes for making it all happen.

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