How many neurons are in the human brain?

The brain contains roughly 86 billion neurons and about 85 billion non-neuronal cells — a ratio of almost exactly one to one, not the ten glia per neuron that textbooks asserted for decades. That older claim, like the bacteria-to-cell ratio, was never based on a measurement. The distribution is counterintuitive: the cerebellum holds about 69 billion of the 86 billion neurons, roughly 80% of them, while the cerebral cortex — where thinking is supposed to happen — holds about 16 billion in a far larger volume.

How the figure is arrived at

The isotropic fractionator, and the method is the point. Instead of counting cells in thin slices and extrapolating — which fails because cell density varies enormously between regions — the tissue is dissolved into a homogeneous suspension of free nuclei. Stir it, take a sample, count nuclei under a microscope, and stain to distinguish neuronal from non-neuronal. Homogenising removes the sampling bias that made every earlier estimate unreliable.

How confident it is

Low estimate

80,000,000,000

High estimate

90,000,000,000

Spread

1.1× between the bounds

Other the human body counts

• How many bones are in the human body? 206 in an adult — about 300 at birth
• How many cells are in the human body? about 30 trillion human cells, plus 38 trillion bacteria
• How many muscles are in the human body? about 600 skeletal muscles
• How many hairs are on your head? about 100,000
• How many taste buds do you have? about 2,000 to 8,000
• How long are all your blood vessels? commonly quoted as 100,000 km — but probably closer to 20,000
• How many base pairs are in human DNA? about 3.1 billion per copy — 6.2 billion in most cells