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It is well established that the brain uses more energy than any other human organ, accounting for up to 20 percent of the body's total haul. Until now, most scientists believed that it used the bulk of that energy to fuel electrical impulses that neurons employ to communicate with one another. Turns out, though, that is only part of the story.
A new study in Proceedings of the National Academy of Sciences USA indicates that two thirds of the brain's energy budget is used to help neurons or nerve cells "fire'' or send signals. The remaining third, however, is used for what study co-author Wei Chen, a radiologist at the University of Minnesota Medical School, refers to as "housekeeping," or cell-health maintenance.
Researchers reached their conclusions after imaging the brain with magnetic resonance spectroscopy (MRS) to measure its energy production during activity shifts. Chen says the technology, which has been around for three decades and is used to track the products of metabolism in different tissues, could prove instrumental one day in detecting brain defects or to diagnose tumors or precursors of neurodegenerative diseases (such as Alzheimer's and Parkinson's) early.
Chen and his colleagues used MRS specifically to track the rate of adenosine triphosphate (ATP) production, the primary source of cellular energy, in rat brains. MRS employs a magnetic resonance imaging (MRI) machine programmed to pick up particular elements in the body—in this case, the three phosphorus atoms in each ATP molecule. Their goal: to determine whether ATP production is linked to brain activity by measuring the energy expended during different levels of consciousness.
Sure enough, ATP levels appeared to vary with brain activity. The team noted that when the lab rats were knocked out, they produced 50 percent fewer ATP molecules than when they were mildly anesthetized.The ATP produced when the brain is inactive, says Chen, seems to go mostly toward cell maintenance, whereas the additional ATP found in the more alert animals fueled other brain functions. He speculates that only a third of the ATP produced in fully awake brains is used for housekeeping functions, leaving the rest for other activities.
"Housekeeping power is important for keeping the brain tissue alive," Chen says, "and for the many biological processes in the brain," in addition to neuronal chats. Charged sodium, calcium and potassium atoms (or ions) are continuously passed through the membranes of cells, so that neurons can recharge to fire. ATP supplies the energy required for these ions to traverse cell membranes. Chen says there must be enough energy to maintain a proper ionic balance inside and outside cells; if too many get stuck inside, it can cause swelling, which can damage cells and lead to strokes and other conditions.
He says the team has since used MRS to study energy demands of a cat's brain, which they said also jumped when the kitty was visually stimulated. Next up: humans, which Chen says researchers hope to study "very soon."