Mechanism of fMRI

SienceDaily (June 19, 2008) — In work that solves a long-standing mystery in neuroscience, researchers at MIT’s Picower Institute for Learning and Memory have shown for the first time that star-shaped brain cells called astrocytes–previously considered bit players by most neuroscientists–make noninvasive brain scans possible.

Imaging techniques such as functional magnetic resonance imaging (fMRI) and positron emission tomography (PET) have transformed neuroscience, providing colorful maps of brain activity in living subjects. The scans’ reds, oranges, yellows and blues represent changes in blood flow and volume triggered by neural activity. But until the MIT study, reported in the June 20 issue of Science, no one knew exactly why this worked.
“Why blood flow is linked to neuronal activity has been a mystery,” said study co-author Mriganka Sur, Sherman Fairchild Professor of Neuroscience and head of the Department of Brain and Cognitive Sciences at MIT. “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.”
Of the two major cell types in the brain, glia outnumber neurons nine to one. Astrocytes–the most common type of glia–extend their branching tendrils both around synapses–through which neurons communicate–and along blood vessels.
Using a cutting-edge technique, Sur and colleagues found that astrocytes receive signals directly from neurons and provide their own neuron-like responses to directly regulate blood flow. They are the missing link between neurons and blood vessels, he said. When astrocytes are shut down, fMRI doesn’t work.

http://www.sciencedaily.com/releases/2008/06/080619142121.htm

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