The human brain has 86 billion neurons, not 100 billion

The adult human brain contains about 86 billion neurons, not the 100 billion long quoted in textbooks, according to the first direct count, published in 2009 by Brazilian neuroscientist Suzana Herculano-Houzel and colleagues.

Where did the 100 billion figure come from?

Nowhere measurable: the round number was never counted. Many articles, reviews and textbooks stated that we have 100 billion neurons and ten times as many glial cells, usually without a reference. When Herculano-Houzel informally surveyed senior neuroscientists in 2007, most took the figure as settled, yet none could name an original source.

Her team counted with the isotropic fractionator, which dissolves brain tissue into a uniform suspension of free-floating cell nuclei that can be stained and sampled. The average adult male brain held about 86.1 billion neurons and 84.6 billion non-neuronal cells: close to one to one, not ten glia per neuron. Herculano-Houzel has suggested that the old idea that neurons are only a tenth of brain cells may have helped feed the myth that we use just 10% of our brains.

Where in the brain are the neurons?

Mostly not in the wrinkled outer layer. The cerebral cortex makes up 82% of brain mass but holds only 19% of the neurons, about 16 billion; around 69 billion sit in the compact cerebellum at the back.

That split explains a puzzle. In 2014 the same lab counted an African elephant brain, about three times the size of ours, and found 257 billion neurons, three times the human total. But 97.5% of them are in the cerebellum. The elephant’s cortex, twice the mass of ours, holds only 5.6 billion neurons, about a third of the human count. So the human brain is not special for its total. It is a scaled-up primate brain, and its edge appears to lie in having more neurons in the cerebral cortex than an elephant or any other primate.