{"schemaVersion":1,"id":"en-mechanism-brain","type":"mechanism","pillar":"body","locale":"en","translationGroupId":"mechanism-brain","slugPath":"body/systems/the-brain","title":"The brain","shortTitle":"The brain","summary":"The most cited and worst described organ. The number everybody repeats is wrong, and most of its neurons are not where you would imagine.","level":"beginner","estimatedMinutes":10,"synonyms":["brain","cerebral cortex","cerebellum","brainstem","lobes of the brain","neurons"],"blocks":[{"id":"b1","type":"heading","level":2,"text":"A number everybody cites, and it is wrong","anchor":"a-number-everybody-cites-and-it-is-wrong"},{"id":"b2","type":"paragraph","content":[{"kind":"text","value":"\"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."}]},{"id":"b3","type":"paragraph","content":[{"kind":"text","value":"The count was eventually done."}]},{"id":"b4","type":"evidence","level":"established","statement":"The adult human brain contains on average around 86 billion neurons, and roughly as many non-neuronal cells.","detail":"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.","referenceIds":["azevedo2009"]},{"id":"b5","type":"paragraph","content":[{"kind":"text","value":"Eighty-six billion is still hard to picture. What matters more is how they are distributed, and that contradicts intuition."}]},{"id":"b6","type":"heading","level":2,"text":"Where the neurons actually are","anchor":"where-the-neurons-actually-are"},{"id":"b7","type":"paragraph","content":[{"kind":"text","value":"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."}]},{"id":"b8","type":"evidence","level":"established","statement":"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.","detail":"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.","referenceIds":["azevedo2009"]},{"id":"b9","type":"paragraph","content":[{"kind":"text","value":"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."}]},{"id":"b10","type":"heading","level":2,"text":"The map, at the coarsest level","anchor":"the-map-at-the-coarsest-level"},{"id":"b11","type":"comparison","title":"The major regions and what is attributed to them","columns":["Region","Position","What is associated with it"],"rows":[{"label":"Cerebral cortex","cells":["Folded outer layer","Conscious perception, decision, language, voluntary command"]},{"label":"Deep structures","cells":["Beneath the cortex","Selecting and initiating movements, memory, emotion"]},{"label":"Cerebellum","cells":["Behind and below","Coordination, timing, correction of movement, balance"]},{"label":"Brainstem","cells":["Between brain and cord","Vital functions: breathing rhythm, heart rate, wakefulness"]}]},{"id":"b12","type":"paragraph","content":[{"kind":"text","value":"The cortex is traditionally divided into four lobes per hemisphere. The "},{"kind":"strong","value":"frontal"},{"kind":"text","value":" lobe occupies the front and carries the motor command among other things; the "},{"kind":"strong","value":"parietal"},{"kind":"text","value":", behind it, receives information from the body; the "},{"kind":"strong","value":"temporal"},{"kind":"text","value":", at the side, takes part in hearing and memory; the "},{"kind":"strong","value":"occipital"},{"kind":"text","value":", at the back, processes vision."}]},{"id":"b13","type":"paragraph","content":[{"kind":"text","value":"The brainstem deserves a special mention in this corpus. It sets the background breathing rhythm the "},{"kind":"link","value":"diaphragm","href":"/en/body/muscles/diaphragm","external":false},{"kind":"text","value":" page describes without naming, and the cardiac acceleration at the start of an effort that the "},{"kind":"link","value":"aerobic system","href":"/en/energy/the-aerobic-system","external":false},{"kind":"text","value":" page observes without saying where it comes from."}]},{"id":"b14","type":"heading","level":2,"text":"What localisation allows you to say, and what it does not","anchor":"what-localisation-allows-you-to-say-and-what-it-does-not"},{"id":"b15","type":"paragraph","content":[{"kind":"text","value":"This is where the page must be careful, because the subject is presented everywhere with a confidence the literature does not support."}]},{"id":"b16","type":"paragraph","content":[{"kind":"text","value":"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."}]},{"id":"b17","type":"callout","tone":"caution","title":"What a brain-imaging picture does not show","content":[[{"kind":"text","value":"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."}],[{"kind":"text","value":"Going from \"this region is more active when doing X\" to \"this region is for X\" is a shortcut the method does not license."}]]},{"id":"b18","type":"heading","level":2,"text":"Three received ideas","anchor":"three-received-ideas"},{"id":"b19","type":"callout","tone":"caution","title":"What the textbooks contradict","content":[[{"kind":"strong","value":"\"We only use 10% of our brain.\""},{"kind":"text","value":" False, and with no identifiable scientific origin. Every region has measurable activity, and none can be damaged without consequence."}],[{"kind":"strong","value":"\"Left brain analytical, right brain creative.\""},{"kind":"text","value":" The two hemispheres are not identical — language is more lateralised than the rest — but nothing supports the existence of personalities \"dominated\" by one side."}],[{"kind":"strong","value":"\"The adult brain is fixed.\""},{"kind":"text","value":" 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."}]]},{"id":"b20","type":"heading","level":2,"text":"Why no scene accompanies this page","anchor":"why-no-scene-accompanies-this-page"},{"id":"b21","type":"callout","tone":"limit","title":"An organ absent from the models","content":[[{"kind":"text","value":"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."}],[{"kind":"text","value":"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."}]]},{"id":"b22","type":"heading","level":2,"text":"What this page does not do","anchor":"what-this-page-does-not-do"},{"id":"b23","type":"callout","tone":"caution","title":"No interpretation, no diagnosis","content":[[{"kind":"text","value":"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."}],[{"kind":"text","value":"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."}]]},{"id":"b24","type":"heading","level":2,"text":"Sources","anchor":"sources"},{"id":"b25","type":"sourceList","title":"Main sources","referenceIds":["kandel2021","purves2018","azevedo2009"]},{"id":"b26","type":"shapierAction","actionId":"conseils-recuperation-nerveuse","label":"Looking after your nervous system day to day","description":"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.","webTarget":"https://shapier.app/en/conseils/recuperation-systeme-nerveux","appTarget":"shapier://conseils/recuperation-systeme-nerveux"},{"id":"b27","type":"quiz","title":"Check my understanding","questions":[{"id":"q1","prompt":"Where does the figure of around 86 billion neurons come from?","choices":[{"id":"q1c1","label":"From a direct count on four adult male brains","correct":true,"explanation":"The tissue was dissolved and neuronal nuclei selectively marked, a direct method where earlier values were extrapolated."},{"id":"q1c2","label":"From an old estimate passed along until it became a fact","correct":false,"explanation":"That is the origin of the hundred billion figure, which never rested on a count; 86 billion comes from an actual count instead."},{"id":"q1c3","label":"From an average established across all primates","correct":false,"explanation":"The study covered four adult human brains; the figure describes a human average, not a comparison between species."}]},{"id":"q2","prompt":"Where are most of the brain's neurons located?","choices":[{"id":"q2c1","label":"In the cerebellum","correct":true,"explanation":"It holds on the order of seventy billion neurons against some fifteen billion for the cerebral cortex, while accounting for a far smaller share of the mass."},{"id":"q2c2","label":"In the cerebral cortex","correct":false,"explanation":"That is what every image depicts, but it holds only a minority of the neurons; cerebellar neurons are much smaller and more tightly packed."},{"id":"q2c3","label":"Spread roughly evenly across the regions","correct":false,"explanation":"Density varies sharply: the cerebellum occupies a tenth of the mass and houses about four fifths of the cells."}]},{"id":"q3","prompt":"What does a 'lit up' area on a brain-imaging picture mean?","choices":[{"id":"q3c1","label":"That it is more active in one condition than in another","correct":true,"explanation":"These images are statistical differences averaged over a group, not proof that the area produces the studied function on its own."},{"id":"q3c2","label":"That it is the only active region during the task","correct":false,"explanation":"The picture shows a difference between two conditions, not a map of absolute activity; every region keeps measurable activity."},{"id":"q3c3","label":"That one can conclude this region is for that function","correct":false,"explanation":"That is precisely the shortcut the method does not license: going from 'more active when doing X' to 'is for X' is not valid."}]}]},{"id":"b28","type":"relatedContent","title":"Read next","targetIds":["en-mechanism-motor-control","en-mechanism-proprioception","en-mechanism-nervous-system"]}],"relations":[{"type":"prerequisite","targetId":"en-mechanism-nervous-system"},{"type":"explains","targetId":"en-mechanism-neuron"},{"type":"next-step","targetId":"en-mechanism-motor-control"},{"type":"explains","targetId":"en-mechanism-sleep-recovery"}],"authors":["equipe-editoriale-shapier"],"reviewers":["Thanh Chau"],"references":[{"id":"kandel2021","authors":"Kandel ER, Koester JD, Mack SH, Siegelbaum SA","year":2021,"title":"Principles of Neural Science, 6th edition","source":"McGraw Hill","kind":"book","url":"https://www.mheducation.com/highered/mhp/product/principles-neural-science-sixth-edition.html?viewOption=student"},{"id":"purves2018","authors":"Purves D, Augustine GJ, Fitzpatrick D, Hall WC, LaMantia AS, Mooney RD, Platt ML, White LE","year":2018,"title":"Neuroscience, 6th edition","source":"Oxford University Press","kind":"book","url":"https://lccn.loc.gov/2017028094"},{"id":"azevedo2009","authors":"Azevedo FAC, Carvalho LRB, Grinberg LT, Farfel JM, Ferretti REL, Leite REP, Jacob Filho W, Lent R, Herculano-Houzel S","year":2009,"title":"Equal numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled-up primate brain","source":"The Journal of Comparative Neurology","kind":"journal-article","doi":"10.1002/cne.21974"}],"review":{"publishedAt":"2026-08-02","reviewedAt":"2026-08-02","reviewDueAt":"2027-08-02","evidenceLevel":"established"},"limitations":["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."],"seo":{"title":"The brain — its map, its real neuron count and three false ideas","description":"Cortex, lobes, cerebellum and brainstem: how the brain is organised, the actual count of its neurons, and what localising functions does not allow you to claim.","canonicalPath":"/en/body/systems/the-brain","image":"/og/en/mechanism-brain.png"},"app":{"offlineEligible":true},"version":1}