How do we know that there are 100 billion neurons in the brain?
The brain is a tangled web of cells, most of which are frustratingly transparent, whose axons are interwoven like a oblong pile of snakes the size of several thousand Olympic swimming pools. Counting them is clearly a significant challenge. Where did that “100 billion” number come from?
It’s a little bit of guesstimation combined with a healthy dose of educated extrapolation. That number, which is now thought to be closer to 86 billion, is complicated by the fact that the brain contains lots of cells that aren’t neurons (like glia). The brain also isn’t uniformly packed with nerve cells, with some areas containing many times more than others.
Old methods relied on staining a slice of brain with a dye that randomly colors nerve cells (like the Golgi method illustrated above by Santiago Ramón y Cajal). You calculate the number in the whole slice, then count slices from different areas of the brain, do some mathematical gymnastics, carry the 2 and get a number like 100 billion.
The newest method, which counted the number of nerve nuclei in a chunk of brain, is much more accurate, but still involves lots of guesswork. But in the end, the number of neurons isn’t nearly as important as how they are organized. An elephant brain weighs more than four times our own, but it’s the map that makes us man.