Ikigai Daily
Menu
WonderBrain fact

The human brain contains about 86 billion neurons, the nerve cells that carry signals, and roughly as many other cells that support them. Each neuron can connect with thousands of others. The figure comes from a 2009 study that counted the cells instead of estimating them.

Azevedo et al., Journal of Comparative Neurology, 2009

MindScience

The story behind the card

For decades textbooks said the brain had 100 billion neurons and ten times as many support cells, called glia, but nobody had actually counted them. In 2009 the Brazilian neuroscientist Suzana Herculano-Houzel and colleagues in Rio de Janeiro used a method she had developed with Roberto Lent, the isotropic fractionator. Brain tissue is dissolved into an even liquid of cell nuclei; a small sample is counted under the microscope, with a stain that marks only neurons, and the result is scaled up. In adult men's brains they found about 86 billion neurons and 85 billion other cells, so the famous ten-to-one ratio was wrong. Another surprise: only about a fifth of the neurons sit in the cerebral cortex, the brain's wrinkled outer layer. Most are packed into the cerebellum, at the lower back of the brain.

Türkçe oku
İnsan beyninde yaklaşık 86 milyar nöron, yani sinyal taşıyan sinir hücresi ve bir o kadar da onları destekleyen başka hücre bulunur. Her nöron binlerce başka nöronla bağlantı kurabilir. Bu sayı, hücreleri tahmin etmek yerine doğrudan sayan 2009 tarihli bir araştırmadan gelir.
Azevedo ve ark., Journal of Comparative Neurology, 2009

Ders kitapları onlarca yıl boyunca beyinde 100 milyar nöron ve bunun on katı kadar glia denen destek hücresi bulunduğunu yazdı; oysa bu hücreleri gerçekten sayan olmamıştı. 2009'da Brezilyalı sinirbilimci Suzana Herculano-Houzel ve Rio de Janeiro'daki çalışma arkadaşları, onun Roberto Lent ile geliştirdiği izotropik fraksiyonlama yöntemini kullandı. Beyin dokusu, hücre çekirdeklerinden oluşan homojen bir sıvıya dönüştürülür; bu sıvıdan alınan küçük bir örnek, yalnızca nöronları boyayan bir işaretleyiciyle mikroskopta sayılır ve sonuç bütüne oranlanır. Yetişkin erkeklerin beyinlerinde yaklaşık 86 milyar nöron ve 85 milyar başka hücre buldular; yani meşhur “on kat” oranı yanlıştı. Bir başka şaşırtıcı sonuç da nöronların ancak beşte birinin beynin kıvrımlı dış katmanı olan serebral kortekste bulunmasıydı. Çoğu, beynin arka alt kısmındaki beyincikte toplanmıştır.

Sources and further reading

Background reading: Brain
Get the mobile appAbout our cards

Related cards

These cards share a subject or reading mode with the card above.

WonderScience

Deliberately smiling can make people feel slightly happier. In 2022 a study of nearly 4,000 people in 19 countries found that posing a smile, or copying a smiling face, raised reported happiness a little; a subtler method, holding a pen between the teeth, gave less clear results.

The idea that the face can feed back into feelings, called the facial feedback hypothesis, goes back to Charles Darwin and William James. In a famous 1988 experiment, Fritz Strack and colleagues had people hold a pen in their teeth, which forces a smile, or between their lips, which prevents one, and those in the first group rated cartoons as funnier. In 2016, seventeen laboratories repeated that experiment exactly and found no effect. A 2019 review of 138 studies concluded that the effect is real but small and varies from study to study. The 2022 Many Smiles Collaboration, led by Nicholas Coles and designed jointly by supporters and sceptics of the idea, found that deliberate smiling could both strengthen and start feelings of happiness, while the pen method remained inconclusive.

Ikigai DailyRead card
WonderScience

Your gut and brain communicate through nerves, hormones, immune signals and microbes, which is why stress and digestion can affect each other.

Scientists call this link the gut–brain axis. The gut has its own network of about 500 million nerve cells, the enteric nervous system, sometimes called the “second brain”, which can run digestion largely by itself. It is connected to the brain mainly through the vagus nerve, a long nerve running from the brainstem down to the abdomen, and most of its fibres carry messages upwards, from the gut to the brain. More than 90 per cent of the body's serotonin, a chemical messenger, is found in the gut. Gut bacteria play a part too: in a 2004 Japanese study, mice raised entirely free of germs overreacted to stress. Stress can speed up or slow down the gut, one reason it often worsens irritable bowel syndrome, a common gut disorder. Most research on the microbes' role has been in animals, and human evidence is still limited.

Ikigai DailyRead card
WonderScience

A placebo is a dummy treatment, such as a sugar pill. In pain studies, placebos can reduce the pain people report and also dampen activity in the brain areas that process pain, even though they contain no active drug.

The word placebo is Latin for “I shall please”. One of the first clues that the effect is physical came in 1978, when Jon Levine and colleagues studied patients recovering from wisdom tooth removal. They gave them a placebo and naloxone, a drug that blocks the effect of endorphins, the body's own painkilling chemicals. In patients who had responded to the placebo, naloxone increased the pain, which suggested that the placebo relief came from endorphins. In 2004 Tor Wager and colleagues used brain scans to show that a placebo painkiller reduced activity in pain-processing regions during a painful stimulus. Expectation works the other way too: in the nocebo effect, from the Latin for “I shall harm”, expecting harm makes symptoms worse. None of this means the pain was imaginary, or that placebos can replace effective treatment.

Ikigai DailyRead card
WonderScience

Memory does not work like a recording. Each time we recall an event, the brain rebuilds it from stored fragments, filling gaps with what we know and what we have heard since, so a memory can change without our noticing.

Psychologists call this reconstructive memory. In his 1932 book Remembering, the British psychologist Frederic Bartlett described asking people to retell “The War of the Ghosts”, a Native American folk tale unfamiliar to them; each retelling drifted towards what felt familiar in their own culture. In 1974 Elizabeth Loftus and John Palmer showed people films of car accidents and asked how fast the cars were going when they “hit” or “smashed” into each other. Those asked with “smashed” gave higher speeds, and a week later were more likely to remember broken glass that was never there. This is known as the misinformation effect. It matters in court: according to the Innocence Project, mistaken eyewitness identification played a part in most US convictions later overturned by DNA evidence.

Ikigai DailyRead card
WonderScience

Predictive processing, an influential theory in neuroscience, says the brain does not simply wait for information from the senses. It keeps guessing what is out there and uses the incoming signals mainly to check and correct those guesses.

The idea goes back to the German physicist Hermann von Helmholtz, who argued in the 1860s that seeing involves “unconscious inference”: the brain fills in what the eyes cannot fully tell it. In 1999 the researchers Rajesh Rao and Dana Ballard turned this into a computer model of vision. Higher brain areas send predictions down, and lower areas send back only the difference between prediction and input, called the prediction error. The philosopher Andy Clark and the neuroscientist Karl Friston later helped develop it into a broad theory of the mind. A well-known illustration is the hollow-mask illusion: the inside of a mask still looks like a face bulging outwards, because the brain's expectation of a normal face outweighs the evidence. The theory is widely used but still debated, and critics ask how some of its claims could be tested.

Ikigai DailyRead card
Next card in the archivePeople usually remember material better when they study it in several sessions spread over time than when they pack the same hours into one sitting. This spacing effect, first measured by the German psychologist Hermann Ebbinghaus in 1885, is one of the most reliable findings in memory research.Read next →