Neurons in a human brain

Image from page 221 of The human body and health : an elementary text-book of essential anatomy, applied...

It is a difficult number to estimate, which is why there is such a large range.

In general, the estimates are done based on calculating “neuron density” and extrapolating that to the volume occupied by the brain. But it is not that simple, because the brain is not a uniform mass of neurons.

Neuron density for different types of neural tissues can be estimated by taking a slice of tissue, putting it under a microscope, and counting individual cells that have been stained for visibility on a marked grid (similar to how white blood cell counts are done). However this type of estimation is specific to the neural tissue being evaluated.

For example, the cerebral cortex is a sheet of neurons with a surface area of 2500 square cm (equivalent to 50×50 cm or 20×20 inches), and 2 – 4 mm thick. So an estimate of neuron density can be made for cortical tissue based on slices, which people put at around 50, 000 cells per cubic mm. [1] Multiplied out gives 25 billion neurons for the cerebral cortex.

This image shows the cerebral cortex how it is usually seen (left), and what it looks likes if “inflated” to form a flat sheet of neurons (right):

The white matter of the brain, just below the 2 mm surface, is almost entirely axons, not neurons.

The cerebellum has very tiny neurons in it, so even though the cerebellum is less than 10% of the brain by volume, some say it has more neurons in it than the rest of the brain combined (50 billion by one estimate).

If one assumes that the cerebellum and the cerebral cortex together account for at least 2/3 of the neurons in the brain, one can then get to an estimate for the whole brain.

But clearly there is a lot of “guestimating” going on.

The mass counting method that Bradley Voytek mentions is an interesting new direction, however it produces a number for the glial (non-neuron) cells that is 10x smaller than the manual method, so it is still controversial whether this method can be trusted.


[1] Harvard ultrascale imaging website analyzing a cubic mm of cerebral cortex. (

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