How many neurons are in the average adult brain?

Answer: About 86 billion.

Cells in the adult brain

One of the more recent estimates of neuronal count puts the number at around 86 billion. This figure is much lower than the early estimates of 100 billion.

Previous estimates relied on staining neurons in one small region of the brain followed by counting the number of neurons observed, and then extrapolating that count to the rest of the brain tissue. However, this approach is likely to produce errors. There is tremendous variance in neuronal density across different brain regions, as some areas of the brain are more dense with neurons than others. The most obvious example is within the cerebellar granule cell layer, which contains several tiny, densely packed granule cells. This approach leads to an extrapolation error.

The updated number comes from a study called “Equal numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled‐up primate brain” published by the Herculano‐Houzel lab in the Journal of Comparative Neurology. They used a homogenized whole brain to come to a conclusion about cell number. Once completely homogenized, the entire sample was stained with DAPI, a marker of DNA. With the DAPI stain, they were able to label all cells. Then, they found that NeuN positively-stained cells, indicative of neurons, is roughly equal to the number of NeuN negatively stained cells, indicative of nonneuronal cells, or presumed glial cells. Using computational imaging software, they counted total numbers of cells in different regions of the brain, and came up with the following numbers:

  • 170.68 ± 13.86 billion cells Among these
  • 85.08 ± 6.92 billion cells are located in the cerebellum
  • 77.18 ± 7.72 in the cerebral cortex
  • 8.42 ± 1.50 in the rest of the brain

Interestingly, they found that these numbers were in line with what was expected from other primates.