The human brain consumes up to 10 times more energy than the rest of the body, eating an average of 20% of our fuel consumption when we are at rest.
Even in comatose patients who are said to be “brain dead,” the brain uses only 2-3 times less energy.
It’s one of great mysteries of human neuroscience: why does a largely inactive organ continue to demand so much power?
A new study finds the answer to a tiny & secret fuel guzzler hidden inside our neurons.
When a brain cell transmits a signal to another neuron, it does so through a synapse or a small space between them.
First, the presynaptic neuron sends a cluster of vesicles to the end of its tail, closest to synapse. These vesicles then suck neuro transmitters from inside the neuron, acting as “envelopes” that hold messages in need of being mailed.
These filled “envelopes” are then transported to the extreme edge of the neuron, where they “dock” & fuse to membrane, releasing their neuro transmitters into the synaptic space.
Once here, these transmitters connect to the receptors of the “post synaptic” cell, thus continuing message.
We already know that the steps of this fundamental process require a significant amount of brain energy, especially when it comes to vesicular fusion. The nerve ends (terminals) closest to synapse cannot store enough energy molecules, which means they have to synthesize them on their own to transmit electrical messages in the brain.
So it makes sense that an active brain consumes a lot of energy. But what happens to this system when neural goes silent & vesicle never attaches to the membrane? Why does the organ continue to consume energy?
To understand this, the researchers designed several nerve terminal experiments, which compared the metabolic state of the synapse when it is active & when it is inactive.
Even when nerve terminal did not firing, the authors found that synaptic vesicles had a high demand for metabolic energy.
The pump responsible for pushing the protons out of the vesicle & therefore sucking up the neuro transmitters never seems to rest. And it requires a constant flow of energy to function.
In fact, this “hidden pump” was responsible for half of the metabolic consumption of resting synapses in experiments.
Indeed, this pump tends to be losses, the researchers say that these synaptic vesicles constantly spilling-out-protons through their pumps, even if they are already full of neurotransmitters & if the neuron is inactive.
“Given the large number of synapses in the human brain & the presence of hundreds of SV to all these nervous terminals, this metabolic cost hidden to quickly return the synapches in a” ready “state arrives at the cost of major & fuel expenditure, likely contributing significantly to the metabolic demands of the brain and its metabolic vulnerability, ”the authors conclude.
More research is needed to understand how different types of neurons can be affected by such high metabolic loads, as they may not all respond in the same way.
Certain neurons in the brain, for example, may be more vulnerable to loss of energy & understand why this could allow us to preserve these messengers, even when deprived of oxygen or sugar.
“These findings help us better understand why the human brain is so vulnerable to disruption or weakening of its fuel supply,” says biochemist Timothy Ryan of Weill Cornell Medicine in New York City.
“If we had a way to safely reduce this energy drain and thereby slow down brain metabolism, it could have significant clinical impact.
The study was published in Science Advances.