A MORE ROBUST ROLE: An M.I.T. professor believes that blood flow may do more than just serve oxygen to neurons in the brain. It may also be key to information processing. Image: © ISTOCKPHOTO/JANNE AHVO
Blood racing through a brain region's web of vessels is a sign that nerve cells in that locale have kicked into action. The blood rushes to active areas to supply firing neurons with the oxygen and glucose they need for energy.
It is this blood flow, which can last up to a minute, that scientists track in functional magnetic resonance imaging (fMRI) to determine which brain areas are responding to different stimuli. But a new theory could pave the way for a reinterpretation of fMRI images, elevating their measurements to the evaluation of actual neuronal processing rather than the subsequent blood flow that indirectly indicates it, and thereby enhancing the fMRI's usefulness in diagnosing neurological problems.
Christopher Moore, an assistant neuroscience professor at the Massachusetts Institute of Technology's McGovern Institute for Brain Research, detailed his hypothesis in a recent article published in the Journal of Neurophysiology. In essence, it suggests blood's role in the cortex (a key brain processing center), specifically, is more than just bringing nutrients to the cell, it can also alter the activity of local neuronal circuits. For instance, in experiments in his lab, Moore has seen that there is more blood flow can arrive in an area that processes information from a presented stimulus to a certain sense (e.g. touch, visual, auditory) prior to the appearance of the stimulus, implying that the flow can prime a circuit for activity, as well.
Researchers estimate that blood flow in areas of the brain increases by 40 percent when neurons start to fire (or send out electronic impulses), whereas the corresponding metabolic rate of the cells only increases by 4 percent, meaning the cell only needs a tenth of the blood it is supplied to reenergize. "[Neuroscientists] call this discrepancy an 'uncoupling' between flow and metabolism," says Kenneth Kwong, an associate professor in radiology at Harvard Medical School and the researcher, along with Seiji Ogawa, who is generally credited with developing fMRI.
Moore believes that the reason for the discrepancy could be that blood not only nourishes cells but may be intimately involved in the information processing.
If true, Moore says, blood should be factored into any model of neuronal processing—how nerve cells in the brain are activated, how impulses are transmitted between them, how long activity lasts, and how it is terminated. In addition to changing what fMRI is actually measuring, such models could potentially provide new clues to causes of enigmatic disorders such as Alzheimer's disease, multiple sclerosis and schziophrenia—potentially paving the way for treatments that involve correcting blood flow as well as (or rather than) chemical deficiencies.
"Historically, fMRI researchers have to be a little apologetic because they're not looking [directly] at the neuron," Moore says. "fMRI would stop being a second-class citizen; instead it would make fMRI a much more interesting tool…a Heisenberg sort of thing [referring to how the act of observing a quantum state changes it], where what you're looking at is actually a part of the computation going on." Further, scans taken over a number of years could help predict neurodegeneration, if vasculature in a particular brain region begins to weaken. Preliminary data already suggest this is the case for many neurological disorders such as schizophrenia.
The so-called hemo-neural hypothesis plays out in three tissue types: neurons, the blood vessels that feed them, and astrocytes, the star-shaped nerve cells that support and maintain neurons. (Astrocytes, the feet of which are splayed on blood vessels, also help maintain the endothelial cells that line the vessels as well as make up the semipermeable blood–brain barrier responsible for keeping chemicals in the blood from seeping into the brain unless they are needed for metabolism or some other function.)