Mechanism
Beginner
10 min read
The brain
The most cited and worst described organ. The number everybody repeats is wrong, and most of its neurons are not where you would imagine.
A number everybody cites, and it is wrong
"A hundred billion neurons." The formula circulates everywhere, including in serious works. It never rested on a count: it was an old estimate, passed along until it became a fact.
The count was eventually done.
Certainty level · Established
The adult human brain contains on average around 86 billion neurons, and roughly as many non-neuronal cells.
The count covers four adult male brains, by dissolving the tissue and selectively marking neuronal nuclei. It is a direct method, where earlier values were extrapolated. Variation between individuals is real — the authors report a spread of several billion — and the figure describes an average, not a person.
Azevedo FAC, Carvalho LRB, Grinberg LT, Farfel JM, Ferretti REL, Leite REP, Jacob Filho W, Lent R, Herculano-Houzel S (2009)
Eighty-six billion is still hard to picture. What matters more is how they are distributed, and that contradicts intuition.
Where the neurons actually are
The cerebral cortex — the folded layer every image depicts — is what people mean when they say "brain". Yet it holds only a minority of the neurons.
Certainty level · Established
The cerebellum holds most of the brain's neurons, markedly more than the cerebral cortex, while accounting for a far smaller share of its mass.
The same study gives the split: on the order of seventy billion for the cerebellum against some fifteen billion for the cerebral cortex. The gap comes from density: cerebellar neurons are much smaller and more tightly packed. The distribution is robust; the values remain averages over a small sample.
Azevedo FAC, Carvalho LRB, Grinberg LT, Farfel JM, Ferretti REL, Leite REP, Jacob Filho W, Lent R, Herculano-Houzel S (2009)
In other words, the structure that coordinates movement, balance and timing occupies a tenth of the mass and houses four fifths of the cells. For a corpus devoted to the moving body, that is far from a detail.
The map, at the coarsest level
The major regions and what is attributed to them
Region
Position
What is associated with it
Cerebral cortex
Folded outer layer
Conscious perception, decision, language, voluntary command
Deep structures
Beneath the cortex
Selecting and initiating movements, memory, emotion
Cerebellum
Behind and below
Coordination, timing, correction of movement, balance
Brainstem
Between brain and cord
Vital functions: breathing rhythm, heart rate, wakefulness
The cortex is traditionally divided into four lobes per hemisphere. The frontal lobe occupies the front and carries the motor command among other things; the parietal, behind it, receives information from the body; the temporal, at the side, takes part in hearing and memory; the occipital, at the back, processes vision.
The brainstem deserves a special mention in this corpus. It sets the background breathing rhythm the diaphragm page describes without naming, and the cardiac acceleration at the start of an effort that the aerobic system page observes without saying where it comes from.
What localisation allows you to say, and what it does not
This is where the page must be careful, because the subject is presented everywhere with a confidence the literature does not support.
Some correspondences are solid: destroying a precise region of the frontal lobe removes the command of part of the body, and the motor map is reproducible between individuals. Others are far looser: complex functions — deciding, remembering, speaking — call on extended networks rather than a zone.
Caution
What a brain-imaging picture does not show
Coloured images of brain activity are statistical differences between two conditions, averaged over a group. A "lit up" area means it is more active in one condition than the other, not that it produces the studied function on its own.
Going from "this region is more active when doing X" to "this region is for X" is a shortcut the method does not license.
Three received ideas
Caution
What the textbooks contradict
"We only use 10% of our brain." False, and with no identifiable scientific origin. Every region has measurable activity, and none can be damaged without consequence.
"Left brain analytical, right brain creative." The two hemispheres are not identical — language is more lateralised than the rest — but nothing supports the existence of personalities "dominated" by one side.
"The adult brain is fixed." Connections change with experience throughout life, and that is what makes any motor learning possible. Whether new neurons are produced in adults, on the other hand, remains a debated subject, and this page does not settle it.
Why no scene accompanies this page
Limit
An organ absent from the models
Body Lab's scenes derive from four sources: the skeleton, the muscles, the joints and the regions of the body. None contains nervous tissue — checking their inventory confirms it. The skull is there, its contents are not.
Showing a skull on a page devoted to the brain would mean showing a container for its contents, which the project forbids itself. The page therefore describes without showing, and says so.
What this page does not do
Caution
No interpretation, no diagnosis
Body Lab describes an anatomy and a general functioning. This page addresses no question of mental health, no neurological disease, and assesses no individual situation.
A sudden disturbance of speech, of vision, of strength on one side of the body or of balance is a medical emergency: the thing to do is call emergency services, not read a page.
Sources
Main sources
- Kandel ER, Koester JD, Mack SH, Siegelbaum SA (2021). Principles of Neural Science, 6th edition. McGraw Hill.
- Purves D, Augustine GJ, Fitzpatrick D, Hall WC, LaMantia AS, Mooney RD, Platt ML, White LE (2018). Neuroscience, 6th edition. Oxford University Press.
- Azevedo FAC, Carvalho LRB, Grinberg LT, Farfel JM, Ferretti REL, Leite REP, Jacob Filho W, Lent R, Herculano-Houzel S (2009). Equal numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled-up primate brain. The Journal of Comparative Neurology.
Put it into practice in Shapier
Looking after your nervous system day to day
Organising sleep and recovery for the nervous system is tracked in Shapier; Body Lab only explains how the brain is organised and what it contains.
Looking after your nervous system day to dayBody Lab explains; Shapier lets you act and track.
Check my understanding
Where does the figure of around 86 billion neurons come from?
From a direct count on four adult male brains
From an old estimate passed along until it became a fact
From an average established across all primates
Where are most of the brain's neurons located?
In the cerebellum
In the cerebral cortex
Spread roughly evenly across the regions
What does a 'lit up' area on a brain-imaging picture mean?
That it is more active in one condition than in another
That it is the only active region during the task
That one can conclude this region is for that function
Choose an answer
Read next
- Motor controlHow an intention becomes a movement. Feedback arrives too late to correct a fast movement, so the nervous system is forced to predict — and a quick movement is launched rather than steered.Described without a scene
- ProprioceptionThe sense that continuously reports the position and tension of the body without your having to look at it. It is what makes walking in the dark possible — and it is not equally reliable for everything it measures.Described without a scene
- The nervous systemThe network that gathers, decides and commands. It works in two directions at once, and a whole part of it escapes the will — otherwise you would have to think about every heartbeat.Described without a scene
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Trust and method
Author
equipe-editoriale-shapier
Editorial review
Thanh Chau
Scientific review
Pending
Published on
August 2, 2026
Reviewed on August 2, 2026
Next review due August 2, 2027
Limits of this page
- No scene accompanies this page: none of the anatomical sources Body Lab uses contains nervous tissue.
- The count cited covers four adult male brains. It gives a solid order of magnitude and a distribution between regions, not an individual value.
- The localisation of functions is described here at the coarsest level. Attributing a precise function to a precise region is markedly more debated than common depictions suggest.
- This page addresses no question of mental health or neurology. A sudden disturbance of speech, vision, strength or balance is a medical emergency.
Sources
- Kandel ER, Koester JD, Mack SH, Siegelbaum SA (2021). Principles of Neural Science, 6th edition. McGraw Hill.
- Purves D, Augustine GJ, Fitzpatrick D, Hall WC, LaMantia AS, Mooney RD, Platt ML, White LE (2018). Neuroscience, 6th edition. Oxford University Press.
- Azevedo FAC, Carvalho LRB, Grinberg LT, Farfel JM, Ferretti REL, Leite REP, Jacob Filho W, Lent R, Herculano-Houzel S (2009). Equal numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled-up primate brain. The Journal of Comparative Neurology.
Educational content. Body Lab does not diagnose and does not replace professional advice.
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