June 20 -- THURSDAY, June 19 (HealthDay News) -- Star-shaped brain cells called astrocytes play a key role in allowing experts to see the many reds, oranges, yellows and blues on brain scans, according to a report by researchers at the Massachusetts Institute of Technology.
Astrocytes, previously thought of as minor players in brain activity, receive signals directly from neurons and provide their own neuron-like responses to directly regulate blood flow. The colors seen on functional magnetic resonance imaging (fMRI) and positron emission tomography (PET), which provide neuroscientists with a map of the brain at work, represent blood flow and volume responding to neural activity.
"Why blood flow is linked to neuronal activity has been a mystery," study co-author Mriganka Sur, head of the department of brain and cognitive sciences at MIT, said in a prepared statement. "Previously, people have argued that the fMRI signal reports local field potentials or waves of incoming electrical activity, but neurons do not connect directly to blood vessels. A causal link between neuronal activity and blood flow has never been shown."
This finding, published in the June 20 issue of Science, shows astrocytes to be the missing link between neurons and blood vessels, he said.
Astrocytes are a common type of glia, one of the two main cell types in the brain. Neurons in the brain are outnumbered nine to one by glia. Astrocytes -- the most common type of glia -- extend their branching tendrils both around synapses -- through which neurons communicate -- and along blood vessels, the researchers said.
Contrary to previous thought, the new report finds that astrocytes influence complex neuronal computations such as the duration and selectivity of brain cell responses to stimuli. Since their chemical signals had rendered them invisible to traditional research methods for monitoring electrical activity, most scientists thought astrocytes did little.
"Electrically, astrocytes are pretty silent," study co-author James Schummers, postdoctoral associate at MIT's Picower Institute for Learning and Memory, said in a prepared statement. "A lot of what we know about neurons is from sticking electrodes in them. We couldn't record from astrocytes, so we ignored them."
The researchers changed this perception by imaging astrocytes with two-photon microscopy. "The first thing we noticed was that the astrocytes were responding to visual stimuli. That took us completely by surprise," Schummers said. "We didn't expect them to do anything at all. Yet there they were, blinking just like neurons were blinking."
"This work shows that astrocytes, which make up 50 percent of the cells in the cortex but whose function was unknown, respond exquisitely to sensory drive, regulate local blood flow in the cortex and even influence neuronal responses," Sur said. "What's more, astrocytes are arranged in orderly feature maps, exquisitely mapped across the cortical surface in sync with neuronal maps."
The MIT researchers next expect to explore exactly how astrocytes work on neurons.
The American Academy of Orthopaedic Surgeons has more about diagnostic imaging.
SOURCE: Massachusetts Institute of Technology, news release, June 19, 2008