Washington, June 20 : An IIT-Kanpur alumnus, currently associated with the Massachusetts Institute of Technology (MIT), says that he and his colleagues have unravelled the mystery as to what makes non-invasive brain scans possible.
Mriganka Sur, Sherman Fairchild Professor of Neuroscience and head of the Department of Brain and Cognitive Sciences at MIT, has revealed that star-shaped brain cells called astrocytes are key to the working of imaging techniques like functional magnetic resonance imaging (fMRI) and positron emission tomography (PET).
Such techniques have transformed neuroscience by providing colourful 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.
Sur points out that no one knew exactly why this worked until the latest MIT study was out.
"Why blood flow is linked to neuronal activity has been a mystery. 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," said Sur, who is a co-author on the study reported in the journal Science.
Astrocytes are the most common type of glia cells in the brain, and they extend their branching tendrils both around synapses through which neurons communicate, and along blood vessels.
Sur and his colleagues used a cutting-edge technique in their study, and found that astrocytes receive signals directly from neurons and provide their own neuron-like responses to directly regulate blood flow.
He said that astrocytes are the missing link between neurons and blood vessels, and that fMRI does not work when astrocytes are shut down.
"Astrocytes are implicated in many brain disorders and express a very large number of genes that are in the brain. Their role is crucial for understanding brain dysfunction as well as for developing potential therapeutics," Sur said.
James Schummers, a postdoctoral associate at MIT's Picower Institute for Learning and Memory who co-authored the study with Sur, said: "Electrically, astrocytes are pretty silent. A lot of what we know about neurons is from sticking electrodes in them. We couldn't record from astrocytes, so we ignored them."
When astrocytes were imaged 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. We didn't know if the rest of the world would think we were crazy," he said.
Sur added: "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. What's more, astrocytes are arranged in orderly feature maps, exquisitely mapped across the cortical surface in sync with neuronal maps."
According to Schummers, the next step of the research team would be to explore exactly how astrocytes work on neurons.