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For more than 50 years, scientists who study the brain have been misled by a squid. Generations of researchers have been working under a false assumption about how our brains use energy. Now, an international team has set the record straight. NPR's Jon Hamilton has the story.

JON HAMILTON: Scientists still use data from squid when the estimate how much energy is consumed by brain cells in people. Squid are easy to study because their brains send some messages along nerve fibers that are so big you can see them with the naked eye.

The enormous never fibers, called giant axons, have made squid the stars of educations films like this one from the 1970s.

(Soundbite of film)

Unidentified Man: This giant axons are about to play a vital role in the squid's life. They not only ensure that the escape response is as fast as possible, but also that all parts of the mantle contract simultaneously, an essential requirement for efficient jet propulsion.

HAMILTON: Early experiments on squid axons showed that it took a lot of energy for a nerve cell to send out a message, and most scientists have assumed this was true in people, too. But a researcher named Henrik Alle didn't buy that.

Mr. HENRIK ALLE (Researcher, Max-Planck Institute): I just accidentally stumbled over this issue when I was preparing tutorials for students.

HAMILTON: Alle is from the Max-Planck Institute in Frankfurt.

Mr. ALLE: I saw this old work. I thought, I cannot believe personally that nature would waste such energy.

HAMILTON: Alle figured that nature would've made the process more efficient in mammals, whose brains send a huge number of messages. So he led a team that measured precisely how much energy was consumed by the axons in rats. And the team reports in the journal Science, that rat axons used only a third as much energy as squid axons. That probably means humans are equally efficient.

But does and of that really matter? Pierre Magistretti, from the Brain Mind Institute in Lausanne, Switzerland, says it does. One reason, he says, has to do with the increasing use of fMRI and PET scans, which show the brain at work.

Mr. PIERRE MAGISTRETTI (Brain Mind Institute): What you see with an fMRI or a PET scan is the amount of energy that a given brain region is consuming to carry out a certain function.

HAMILTON: To fully understand what those scans mean, you have to know precisely where the energy is going. The squid model assumes most of the energy for brain communication is consumed by the brain cell sending a message. But the new research suggests that the real energy drain occurs somewhere else — probably where the message gets transferred to another cell. And Magistretti says that can be a long way from the place where the message originated.

Mr. MAGISTRETTI: Imagine if you take a neuron that is in the spinal cord and controls the contraction of your feet muscle, you can imagine that this axon will be almost one meter long.

HAMILTON: That's unusual though. In the brain, the distance traveled by most messages is microscopic. As a result, the new finding doesn't affect the results of most fMRI studies.

But Marcus Raichle, who does a lot of brain imaging at Washington University in St. Louis, says the new research has given him something to think about.

Mr. MARCUS RAICHLE (Washington University): There is always this tendency, that if you're working in an area and your experiments are working well and you're getting good data and so forth, to not think of the larger context in which this is occurring.

HAMILTON: Raichle says the study is also a reminder that although we know 20 percent of a human's energy goes to power the brain, we still have a lot to learn about the processes consuming all that energy.

Jon Hamilton, NPR News.

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