The human brain consumes up to ten times the energy that the rest of the body does, consuming approximately 20% of our fuel intake while we sleep.
Even with comatose individuals claimed to be ‘brain dead,’ the brain consumes just two to three times as much energy.
Why does an entirely dormant organ requires so much power is one of the great puzzles of human neurology.
A recent study identifies the answer source as a tiny and hidden fuel-guzzler hidden within our brains.
When one neuron transmits information to another, it does so via a synapse or a small space between them.
To begin, the presynaptic neuron delivers a cluster of vesicles to the tip of its tail, closest to the synaptic vesicle. These vesicles subsequently absorb neurotransmitters from within the cell, working similarly to ‘envelopes’ that contain messages awaiting delivery.
These ‘envelopes’ are then taken to the neuron’s edge, where they dock and fuse with the membrane, releasing their neurotransmitters into the synaptic gap.
When these transmitters reach this place, they attach to receptors on the ‘post-synaptic’ cell, completing the message.
We already know that the stages involved in this fundamental process take a substantial amount of brain energy, most notably during vesicle fusion. Due to the fact that the nerve ends (terminals) nearest to the synapse lack sufficient energy molecules to conduct electrical messages in the brain, they must synthesize them independently.
As a result, it’s unsurprising that a functioning brain needs so much energy. However, what happens when neuronal firing ceases and the vesicle never docks to the membrane? Why does the organ continue to consume so much power?
To ascertain this, researchers conducted a series of tests on nerve terminals, comparing the metabolic status of the synapse during active and inactive states.
Even when nerve terminals were dormant, the authors determined that synaptic vesicles consumed a significant amount of metabolic energy.
The pump that is responsible for releasing protons from the vesicle and attracting neurotransmitters appears to be constantly active. Additionally, it requires an uninterrupted source of electricity to operate.
Indeed, this ‘secret’ pump accounted for half of the resting synapse’s metabolic use in tests.
This is because this pump is prone to leakage, according to the researchers. As a result, synaptic vesicles continuously leak protons via their pumps, even if they are already complete with neurotransmitters and the neuron is dormant.
“Given a large number of synapses in the human brain and the presence of hundreds of SVs at each of these nerve terminals,” the authors conclude, “this hidden metabolic cost of rapidly returning synapses to a’ready’ state likely contributes significantly to the brain’s metabolic demands and metabolic vulnerability.”
Additional research is needed to discover how such massive metabolic loads affect different types of neurons, as they may not behave similarly.
Specific neurons in the brain, for example, maybe more susceptible to energy loss, and understanding why may allow us to sustain these messages even when oxygen or sugar are depleted.”
These discoveries shed light on why the human brain is so susceptible to interruption or deterioration of its fuel supply, “Timothy Ryan, a biochemist at Weill Cornell Medicine in New York City, agrees.
“If we could safely reduce this energy drain and thereby slow brain metabolism, it would have a significant clinical impact.”