Introducing the brain: its energy use (Introduction)

by David Turell , Wednesday, June 04, 2025, 19:17 (8 days ago) @ David Turell

It is on all the time:

https://www.quantamagazine.org/how-much-energy-does-it-take-to-think-20250604/

"It often feels as though we allocate our mental energy through strenuous attention and focus. But the new research builds on a growing understanding that the majority of the brain’s function goes to maintenance. While many neuroscientists have historically focused on active, outward cognition, such as attention, problem-solving, working memory and decision-making, it’s becoming clear that beneath the surface, our background processing is a hidden hive of activity. Our brains regulate our bodies’ key physiological systems, allocating resources where they’re needed as we consciously and subconsciously react to the demands of our ever-changing environments.

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"The human brain is incredibly expensive to run. At roughly 2% of body weight, the organ gorges on 20% of our body’s energetic resources. “It’s hugely metabolically demanding,” Jamadar said. For infants, that number is closer to 50%.

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"The brain’s energy comes in the form of the molecule adenosine triphosphate (ATP), which cells make from glucose and oxygen. A tremendous expanse of thin capillaries — an estimated 400 miles of vascular wiring — weaves through brain tissue to carry glucose- and oxygen-rich blood to neurons and other brain cells. Once synthesized within cells, ATP powers communication between neurons, which enact the brain’s functions. Neurons carry electrical signals to their synapses, which allow the cells to exchange molecular messages; the strength of a signal determines whether they will release molecules (or “fire”). If they do, that molecular signal determines whether the next neuron will pass on the message, and so on. Maintaining what are known as membrane potentials — stable voltages across a neuron’s membrane that ensure that the cell is primed to fire when called upon — is known to account for at least half of the brain’s total energy budget.

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"When performed simultaneously, PET and fMRI can provide complementary information on how glucose is being consumed by the brain, Jamadar said. It’s not a complete measure of the brain’s energy use because neural tissues can also convert some amino acids(opens a new tab) into ATP, but the vast majority of the brain’s ATP is produced by glucose metabolism.

"Jamadar’s analysis showed that a brain performing active tasks consumes just 5% more energy compared to a resting brain. When we are engaged in an effortful, goal-directed task, such as studying a bus schedule in a new city, neuronal firing rates increase in the relevant brain regions or networks — in that example, visual and language processing regions. This accounts for that extra 5%; the remaining 95% goes to the brain’s base metabolic load.

"Researchers don’t know precisely how that load is allocated, but over the past few decades, they have clarified what the brain is doing in the background. “Around the mid-’90s we started to realize as a discipline [that] actually there is a whole heap of stuff happening when someone is lying there at rest and they’re not explicitly engaged in a task,” she said. “We used to think about ongoing resting activity that is not related to the task at hand as noise, but now we know that there is a lot of signal in that noise.”

"Much of that signal is from the default mode network, which operates while we’re resting or otherwise not engaged in apparent activity. This network is involved in the mental experience of drifting between past, present and future scenarios — what you might make for dinner, a memory from last week, some pain in your hip. Additionally, beneath the iceberg of awareness, our brains are keeping track of the mosaic of physical variables — body temperature, blood glucose level, heart rate, respiration, and so on — that must remain stable, in a state known as homeostasis, to keep us alive. If any of them stray too far, things can get bad pretty quickly.

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"A 5% increased energy demand during active thought may not sound like much, but in the context of the entire body and the energy-hungry brain, it can add up. And when you consider the strict energetic constraints our ancestors had to deal with, weariness at the end of a taxing day suddenly makes a lot more sense.

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"To better understand these energetic constraints, in 2023 Padamsey summarized research on certain peculiarities of electrical signaling that indicate an evolutionary tendency toward energy efficiency. For one thing, you might imagine that the faster you transmit information, the better. But the brain’s optimal transmission rate is far lower than might be expected.

"Theoretically, the top speed for a neuron to feasibly fire and send information to its neighbor is 500 hertz. However, if neurons actually fired at 500 hertz, the system would become completely overwhelmed. The optimal information rate(opens a new tab) — the fastest rate at which neurons can still distinguish messages from their neighbors — is half that, or 250 hertz."

Comment: an overall constraint on burn rate is logical. The energy use is well known.