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| Neurons: the human brain has about 1 trillion of them! |
The brain is made up of about 1 trillion tiny nerve cells called neurons. When the brain is working, these neurons send and receive signals to each other. The more they send and receive signals, the more oxygen they need from the blood flowing around in the brain. This means that areas of the brain with heavily signaling neurons get more blood flow. fMRI uses a magnetic field to measure increases in blood flow all over the brain. This way, the fMRI scan allows us to see what areas of the brain are working harder than others.
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