Brains also have supply chain issues – blood flows where it can, and neurons must make do with what they get
Neuroscientists have long assumed that neurons are greedy, hungry units that demand more energy when they become more active, and the circulatory system complies by providing as much blood as they require to fuel their activity. Indeed, as neuronal activity increases in response to a task, blood flow to that part of the brain increases even more than its rate of energy use, leading to a surplus. This increase is the basis of common functional imaging technology that generates colored maps of brain activity.
Scientists used to interpret this apparent mismatch in blood flow and energy demand as evidence that there is no shortage of blood supply to the brain. The idea of a nonlimited supply was based on the observation that only about 40% of the oxygen delivered to each part of the brain is used – and this percentage actually drops as parts of the brain become more active. It seemed to make evolutionary sense: The brain would have evolved this faster-than-needed increase in blood flow as a safety feature that guarantees sufficient oxygen delivery at all times.
But does blood distribution in the brain actually support a demand-based system? As a neuroscientist myself, I had previously examined a number of other assumptions about the most basic facts about brains and found that they didn’t pan out. To name a few: Human brains don’t have 100 billion neurons, though they do have the most cortical neurons of any species; the degree of folding of the cerebral cortex does not indicate how many neurons are present; and it’s not larger animals that live longer, but those with more neurons in their cortex.