How Does the Brain Use So Much Energy? Not in Electrical Signals.

By Eliza Strickland | September 14, 2009 5:46 pm

neuronsExperiments conducted on squid brains in the early days of neuroscience created misunderstandings about the workings of the human brain that have persisted for 70 years, according to a new study. While the squid experiments did shed light on how messages are transmitted between brain cells with electrochemical signals (and led to a Nobel Prize for the experimenters), researchers are just now realizing that the results gave scientists a confused idea about the efficiency of neurons.

The story begins seventy years ago when a pair of British physiologists, Alan Hodgkin and Andrew Huxley, took the first stab at figuring out how neurons transmit electrical signals, known as action potentials. Because most neurons are small–in humans, a cubic millimeter of gray matter can contain 40,000 neurons–the duo turned to squid, which contain a giant axon, the long thin part of a neuron through which action potentials travel [ScienceNOW Daily News]. Those early experiments found that transmitting the action potential along the axon was a very inefficient process that used a great deal of energy, and neuroscientists ever since have assumed that mammal brains had the same inefficient wiring.

Researcher Henrik Alle, lead author of the new study published in Science, decided to reexamine the old assumptions. “I saw this old work,” says Alle. “I thought I cannot believe personally that nature would waste such energy.” Alle figured that nature would have made the process more efficient in mammals, whose brains send a huge number of messages [NPR News].

Alle and his colleagues studied rat brains using sophisticated techniques that weren’t available to Hodgkin and Huxley, and found that rat neurons use only about a third as much energy to transmit the action potential. The researchers say we can assume that the results from rats can be applied to human brain cells. “Electrical signals found in mammalian brain cell types are very similar” [New Scientist], says Alle.

The difference between the cephalopod and the mammals can be explained by the movements of the positively and negatively charged ions that flow in and out of the neuron, changing its voltage and beginning the electric pulse of the action potential that moves down the axon. Hodgkin and Huxley were the first to suggest that the squid cells were inefficient because sodium ions entering the cells neutralised the effect of potassium ions leaving. This hampered the creation of a net voltage across the cell membrane. “It’s like having the accelerator and the brake on at the same time,” says Arnd Roth [New Scientist], a study coauthor. In rat cells, however, the process is better coordinated so that almost all the sodium ions enter before potassium ions rush out.

The results don’t change the scientific thesis that although the brain accounts for only 2 percent of our body weight, it consumed 20 percent of our energy–it just means that the energy is being used by the neurons in other ways than to generate action potentials. Researchers suspect that the bulk of energy that goes to the brain is used for keeping the brain cells alive and used in synapses, where signals are transmitted from one neuron to the next.

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Image: iStockphoto

CATEGORIZED UNDER: Feature, Mind & Brain
MORE ABOUT: neurons, Nobel Prize
  • NewEnglandBob

    …squid cells were inefficient because sodium ions entering the cells neutralised the effect of potassium ions leaving. This hampered the creation of a net voltage across the cell membrane. “It’s like having the accelerator and the brake on at the same time,” says Arnd Roth

    Uh Oh, PZ Myers will not like this one bit.

  • Sven DiMilo

    This doesn’t make any sense. Action potentials are passive processes–no energy is used in firing an action potential in squid or human neurons. The energy is used to pump the ions that have moved passively across the membrane back where they belong. This happens (mostly) when the neuron is “resting” between APs. There are a number of reasons why this resetting might be more efficient in mammals–smaller axon diameter and myelination, to name just two.

  • Carman

    @sven dimilo: And if you read the actual article or even Science’s nontechnical version, you’ll find that is exactly what it says. Really fundamental mis-statement by the author of this article.

  • Brian

    Re: “…and neuroscientists ever since have assumed that mammal brains had the same…” versus “…researchers say we can assume that the results from rats can be applied to human brain cells…”.

    Um, wasn’t it their assumptions that led Hodgkin and Huxley (or perhaps their successors) astray? Is making a new assumption wise under the circumstances? I totally get that there’s a reason why Alle would think that this could be extrapolated to all mammals. However when you’re trying to correct a methodological error of your predecessors, it’s not wise to use the same foundational methodology!

    Maybe Alle’s team should get oh, say a half-dozen mammal results (all from different species please) before we extrapolate to all mammals. I’m just sayin’.

  • anti_supernaturalist

    Two brief comments on “old” research:

    1. it’s vital (not just useful) to question “old” research findings especially if they bear on a topic of contemporary importance. If our so-called “knowledge” doesn’t change under the impact of new theories, methods, new information, and good old-fashioned skepticism, the job of science ain’t gittin’ done rite.

    2. We have no excuse for smugness when we sift our ancestors beliefs — all we can ask is whether their advances outshine their “failures” to be as ”enlightened” as we are. In less than 70 years, we will be someone’s ignorant ancestors.

    We’ve retained plenty of our ancestors’ shortcomings — warfare, religion, child prostitution, poverty, state capitalism, adulation of the rich, racism . . . in addition to outdated scientific “facts,” conjectures, and theories.

    the anti_supernaturalist

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