About how many neurons are packed into one adult human brain?
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Answer
About 86 billion neurons
For decades textbooks said 100 billion. The better figure came from a Brazilian lab that dissolved brains into a kind of soup and counted the cell nuclei.
How to estimate it
A brain weighs 1.4 kg, so its volume is about 1,400 cubic centimetres. Suppose each neuron, with its share of wiring and support cells, fills a cube 25 micrometres wide. That cube is 0.0025 cm on a side, or 1.6 × 10^-8 cubic centimetres. 1,400 / 1.6 × 10^-8 gives about 90 billion. The best count is about 86 billion neurons.
Source: Azevedo et al. 2009, Journal of Comparative Neurology
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