- Published1 Feb 2019
- Reviewed1 Feb 2019
Michael W. Richardson
Like all the other parts of your body, your brain needs energy to operate. Befitting the unique role the brain plays in our bodies it also has unique needs and uses for that energy. Simon Laughlin, professor in the department of zoology at Cambridge University, explains why the brain needs so much energy, how it uses energy, and how those needs have affected our evolution.
How much energy does the brain require?
For the average adult in a resting state, the brain consumes about 20 percent of the body’s energy. The brain’s primary function — processing and transmitting information through electrical signals — is very, very expensive in terms of energy use.
The exact percentages are difficult to ascertain, but we have pretty good estimates of where that energy is going, though it varies by the area of the brain. In the cerebral cortex of mice, about a quarter of the brain’s energy goes to maintaining the neurons and glial cells themselves — the processes that all cells go through to remain alive. The remaining 75 percent is used for signaling — sending and processing electrical signals across the brain’s circuits. These numbers seem to be very similar in humans.
The bulk of that energy is consumed at the synapses — the tiny gaps between brain cells where signals are sent and received. There, the cells are steadily pumping ions into the gap between cells — exchanging potassium and sodium to create electrical charges. This pumping action is fundamental to the operation of brain circuits, but they are very energy intensive.
Do certain parts or areas of the brain require more energy than others?
Between the two major types of tissue in the brain — gray matter and white matter — gray matter requires far more energy than white matter. White matter, made up of bundles of axons, contains large amounts of myelin, the fatty substance that wraps around axons to insulate them and keep electricity from leaking out. Because of this insulation, white matter uses about 20–25 percent as much energy as gray matter, which is made up of dendrites, cell bodies, and the sites of synapses.
Certain functions require more energy than others. The brain areas responsible for auditory processing require more energy than the olfactory system or the areas of the brain responsible for memory. Hearing requires very fast and precise signaling — it wouldn’t be advantageous for the sound of danger to be delayed in any way. Relatively slow processes like smell don’t have the same intense energy needs.
Do you use more energy when thinking hard or completing complex tasks?
There are two important factors to keep in mind. The brain requires this expensive electrical power to operate. And your brain never shuts off. Even when you’re sleeping at night, the brain consumes roughly as much energy as it does during the day. While you rest, your neurons are constantly communicating, updating each other on what is happening. Their constant vigilance is where the bulk of the energy is consumed.
If a specific task requires a certain area of the brain, the area’s energy needs do increase. You can see the increase on an fMRI scan — the area will be bright red where the circuits are especially active. If you’re speaking to another person Broca’s area — the area associated with creating speech —will become more active. Despite what you might assume from the bright colors, the energy increase is minor — about eight percent at most. Unlike muscles in your leg, where energy consumption can increase three to four times when you’re walking compared to when you’re sitting, the brain requires a relatively steady amount — whether you’re doing something complex like solving a difficult math problem or staring into space. The increase is minor, compared to the large amount of energy it needs as a baseline.
There’s a myth that you only use 10 percent of your brain, and a magic pill could unlock the remaining 90 percent. The bulk of your neurons are relatively silent for long stretches of time, waiting to spring into action when activated. But they’re doing so to remain energy efficient. If you somehow tripled the number of neurons activated at one time, the brain’s oxygen needs would increase dramatically, perhaps using as much as your leg muscles would while sprinting.
Has the brain’s energy needs affected human evolution?
The brain doesn’t have a reserve of energy to store away for when it needs it. Unlike muscles, which can store excess carbohydrates, the brain needs to be constantly supplied with oxygen and energy in order to run properly. If the blood supply to the brain is cut off or disrupted — like during a stroke or head injury — neurons start shutting down quickly. This may seem like a flaw, but it is integral to the working of the brain. If the brain contained cells that stored backup power, those cells would take up space in between neurons. This would increase the lengths that electrical signals would have to travel, and they would need more energy to do so. Early organisms may have had nervous systems that included these kinds of fail-safes, but over millions of years of evolution, we sacrificed this backup power for efficiency. It makes us susceptible to injury, but it also allows us to take advantage of the brain’s complex circuitry.
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