How the brain leaks energy

I think mental energy is best described with this viral message:

“My brain has too many tabs open,

4 of them are frozen,

and I have no idea where the music is coming from”

We all know the feeling. It’s clear that we don’t have unlimited mental resources. We need sleep, we need downtime, and we need frequent opportunities to recharge our brain’s battery. 

But, why? Before we can answer that, we need to cover some neuroscience 101. 

Brain cells communicate with each other through both electrical and chemical signals. Chemical signals are conveyed with messengers called neurotransmitters. You’ve certainly heard of some of these: GABA, serotonin, and dopamine, for example. These neurotransmitters are packaged in tiny sacs, or vesicles, that are released by a brain cell into the synapse (the physical gap between cells) and then received by the next brain cell. It seems obvious that this process of transferring the message requires energy, or ATP, right? Right. When we get a bit more granular, we see that this movement of vesicles is managed by pumps. 

Now, here’s where it gets interesting. Of course the active neurons, the ones firing and communicating with each other, use a lot of energy. But it turns out, as a recent study revealed, that the inactive neurons, the ones that aren’t firing, also use a lot of energy. In fact, this seems to be due to largely one pump, in particular. It’s called a proton pump and it accounts for a whopping 44% of all energy usage within the resting synapse. 

Why the heck is it using so much energy when the synapse is at rest? 

Well, the synapses are actually always primed. The brain makes sure vesicles filled with neurotransmitters are always at the ready. That’s actually a good thing and we likely evolved this way so that we can spring into action whenever we need to. 

But, it goes deeper. Even after lots of extra vesicles are full of neurotransmitters - we’re calling that being primed - there are other pumps, called transporter proteins, that keep working even when they aren’t carrying any neurotransmitters. This, in turn, forces the proton pump to keep working. So, there’s energy being burned even though nothing “productive” is happening. And that, my friends, is the source of energy leakages.

To put it another way, the synapses in the brain function like a car sitting idle. Gas is being burned, even though the car isn’t going anywhere. But, that’s the tradeoff for being able to rev the engine and get on your way very quickly.

Now, this is only part of the puzzle and certainly doesn’t entirely account for the large fluxes of mental energy we experience. But, it’s really cool and can help us appreciate the complexity of our mental energy. It will also be used to better understand brain diseases that involve reduced metabolism, such as Alzheimer’s and Parkinson’s.

So, let’s offer a bit of gratitude to our brain. Dear brain, although you’re the most maintenance organ and require 20% of our body’s energy to function, we thank you for always being on in at least some capacity. We promise, however, not to take advantage of your “ready to go” vibe all the time. We will rest, we will take time to recover, and we will gift you moments of quiet so that your reserves can be restored. You just keep doing you.