What’s the source of energy that powers the human brain?Fabian van den Berg:
Mainly sugar, glucose. Glucose is broken down and in the process it is used to create a gradient of hydrogen ions (oxygen is also needed, which is why we breathe). So you get a sort of dam with lots of hydrogen on one side and little on the other. They can only pass through a specific structure, which uses the force to create ATP. ATP is then used to power many biological processes, including the active processes that keep your neurons firing. (See Fabian van den Berg's answer to "Why do we inhale oxygen and exhale carbon dioxide?" for more details.)
Neurons fire using a similar method of gradients. When the incoming signal increases the voltage enough, voltage gated channels spring open and sodium can flow in making it more positive. The neighboring gate also springs open, the same happens, and the chain keeps going until the synapses, where neurotransmitters are released to, do the same to the next neuron (see Fabian van den Berg's answer to "How does opening a sodium channel cause depolarization in a neuron?" for a better description).
In order to keep the neuron negative, and to reset it back after firing, an active process ferries ions across the membrane, which requires energy, and the energy comes from ATP.
So ultimately your brain runs on sugar and oxygen, which it uses to power the ATP machine in your mitochondria.
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