{"schemaVersion":1,"id":"en-mechanism-nervous-system","type":"mechanism","pillar":"body","locale":"en","translationGroupId":"mechanism-nervous-system","slugPath":"body/systems/the-nervous-system","title":"The nervous system","shortTitle":"Nervous system","summary":"The 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.","level":"beginner","estimatedMinutes":9,"synonyms":["nervous system","central nervous system","peripheral nervous system","nerves","spinal cord","autonomic nervous system"],"blocks":[{"id":"b1","type":"heading","level":2,"text":"The system the corpus named without describing","anchor":"the-system-the-corpus-named-without-describing"},{"id":"b2","type":"paragraph","content":[{"kind":"text","value":"Twenty-six pages of Body Lab mention a nerve. The "},{"kind":"link","value":"motor command","href":"/en/adaptations/motor-command","external":false},{"kind":"text","value":" page describes how muscle is recruited, and every muscle page gives its nerve supply. But none said what the system carrying all this is, nor how it is organised."}]},{"id":"b3","type":"paragraph","content":[{"kind":"text","value":"This is the first page in a series on the systems of the body, and it starts with the most cross-cutting one: the system that links all the others."}]},{"id":"b4","type":"callout","tone":"insight","title":"The answer in three sentences","content":[[{"kind":"text","value":"The nervous system gathers information, processes it and sends commands. It is built as two anatomical sets — a centre protected by bone, and a network running from it to the whole body — and works in two directions at once."}],[{"kind":"text","value":"A whole part of it works without the will, and that is indispensable: nobody could think about every heartbeat."}]]},{"id":"b5","type":"heading","level":2,"text":"Two sets, one continuity","anchor":"two-sets-one-continuity"},{"id":"b6","type":"definition","term":"Central nervous system","definition":"Brain and spinal cord, both housed in a bony casing — the skull for one, the vertebral canal for the other. This is where information is processed and commands are formed.","alsoKnownAs":["CNS"]},{"id":"b7","type":"definition","term":"Peripheral nervous system","definition":"All the nerves running from the centre to the skin, the muscles, the joints and the organs. They are cables: they carry, they do not decide.","alsoKnownAs":["PNS"]},{"id":"b8","type":"paragraph","content":[{"kind":"text","value":"The separation is anatomical, not functional. A signal does not stop at the border: it crosses. What changes is the environment — bony protection, capacity for repair, cell organisation — not the nature of the message."}]},{"id":"b9","type":"paragraph","content":[{"kind":"text","value":"The "},{"kind":"link","value":"spinal cord","href":"/en/body/joints/lumbar-spine","external":false},{"kind":"text","value":" occupies the canal of the vertebrae and ends around the top of the lumbar region. Below that, nerve roots descend, which explains the arrangement of the nerves of the lower limb."}]},{"id":"b10","type":"heading","level":2,"text":"Two directions, at the same time","anchor":"two-directions-at-the-same-time"},{"id":"b11","type":"paragraph","content":[{"kind":"text","value":"This is the point common depictions skip: the nervous system does not run from the brain to the body. It runs both ways at once, and the incoming flow is larger than the outgoing one."}]},{"id":"b12","type":"comparison","title":"What travels, and in which direction","columns":["Pathway","Direction","What it carries","Example"],"rows":[{"label":"Sensory","cells":["From the body to the centre","Touch, position, tension, temperature","Feeling a foot slip"]},{"label":"Motor","cells":["From the centre to the body","Commands to contract","Catching the slip"]},{"label":"Autonomic","cells":["Both ways","Non-voluntary settings","Speeding the heart under effort"]}]},{"id":"b13","type":"paragraph","content":[{"kind":"text","value":"The sensory pathway includes a channel ordinary life never notices: "},{"kind":"term","value":"proprioception","definition":"continuous information about the position and tension of the body's segments"},{"kind":"text","value":", which reports constantly on where the limbs are without your having to look at them. This is the channel that makes it possible to walk in the dark, or to hold a squat without seeing yourself."}]},{"id":"b14","type":"heading","level":2,"text":"What does not go through a decision","anchor":"what-does-not-go-through-a-decision"},{"id":"b15","type":"definition","term":"Autonomic nervous system","definition":"The part of the nervous system that sets what is not decided: heart rate, vessel diameter, background breathing rhythm, digestion, sweating. It runs permanently, including during sleep.","alsoKnownAs":["vegetative system"]},{"id":"b16","type":"paragraph","content":[{"kind":"text","value":"It is traditionally described as two complementary sides: one that prepares for effort — the heart speeds up, the vessels of the muscles open, digestion slows — and one that favours rest and recovery."}]},{"id":"b17","type":"paragraph","content":[{"kind":"text","value":"That division illuminates several pages in this corpus. When the "},{"kind":"link","value":"aerobic system","href":"/en/energy/the-aerobic-system","external":false},{"kind":"text","value":" page describes breathing and cardiac output rising at the start of an effort, this is the system acting, and nobody commands it."}]},{"id":"b18","type":"callout","tone":"caution","title":"A nuance popular accounts erase","content":[[{"kind":"text","value":"The two sides are often presented as a switch: one turns on, the other turns off. The reality is a continuous setting where both are permanently active to varying degrees."}],[{"kind":"text","value":"Claims that a simple technique can \"activate\" one side go well beyond what physiology establishes."}]]},{"id":"b19","type":"heading","level":2,"text":"Fast, but not instant","anchor":"fast-but-not-instant"},{"id":"b20","type":"paragraph","content":[{"kind":"text","value":"A nerve signal takes time to travel along a nerve, and that time depends on the calibre of the fibre."}]},{"id":"b21","type":"evidence","level":"established","statement":"The conduction speed of a nerve fibre depends on its diameter and on the presence of a myelin sheath: it ranges from around a metre per second for the finest fibres to more than a hundred metres per second for the thickest and best sheathed.","detail":"These values are measured by stimulation and recording on fibres of different calibres, and are reproduced across the reference works in neuroscience. They describe categories of fibre, not a particular nerve in a given person.","referenceIds":["kandel2021","purves2018"]},{"id":"b22","type":"paragraph","content":[{"kind":"text","value":"That difference has a consequence everybody has already experienced. A reflex — pulling a hand back from a hot surface, catching a foot that slips — fires before you are aware of what is happening. The route is shorter: the information enters the spinal cord and leaves it again without climbing to the brain, which is informed afterwards."}]},{"id":"b23","type":"paragraph","content":[{"kind":"text","value":"In other words, in certain situations the body has already acted by the time you understand. That is not a figure of speech, it is a difference in circuit length."}]},{"id":"b24","type":"heading","level":2,"text":"Why no scene accompanies this page","anchor":"why-no-scene-accompanies-this-page"},{"id":"b25","type":"callout","tone":"limit","title":"A system absent from the models","content":[[{"kind":"text","value":"Body Lab's scenes derive from anatomical sources supplying four sets: the skeleton, the muscles, the joints and the regions of the body. No nerve, no ganglion, no spinal cord exists there as a distinct part — checking the inventory confirms it."}],[{"kind":"text","value":"This page therefore joins "},{"kind":"link","value":"fascia","href":"/en/body/fascia","external":false},{"kind":"text","value":", "},{"kind":"link","value":"cartilage","href":"/en/body/cartilage","external":false},{"kind":"text","value":" and "},{"kind":"link","value":"dietary fat","href":"/en/energy/what-happens-to-dietary-fat","external":false},{"kind":"text","value":": the corpus describes them without being able to show them, and says so rather than attaching the page to a scene that is about something else."}],[{"kind":"text","value":"The "},{"kind":"link","value":"Body","href":"/en/body","external":false},{"kind":"text","value":" hub did announce that this system would not be covered, for want of being able to show it. The position has changed on one point: being unable to show a system does not require saying nothing about it, as long as the absence of a model is declared."}]]},{"id":"b26","type":"heading","level":2,"text":"What this page does not do","anchor":"what-this-page-does-not-do"},{"id":"b27","type":"callout","tone":"caution","title":"No assessment, no diagnosis","content":[[{"kind":"text","value":"Body Lab describes general mechanisms. This page allows no symptom to be interpreted and assesses no individual situation."}],[{"kind":"text","value":"Numbness, loss of strength, loss of sensation, difficulty speaking or moving, or a sudden disturbance of balance belongs to a health professional, without delay. These are situations where time matters, and no page replaces an examination."}]]},{"id":"b28","type":"heading","level":2,"text":"Sources","anchor":"sources"},{"id":"b29","type":"sourceList","title":"Main sources","referenceIds":["kandel2021","purves2018","standring2020"]},{"id":"b30","type":"shapierAction","actionId":"nervous-system-recovery","label":"Track your nervous system recovery","description":"Tracking nervous fatigue and adjusting recovery between sessions happens in Shapier; Body Lab only explains how this system gathers, decides and commands.","webTarget":"https://shapier.app/en/conseils/recuperation-systeme-nerveux","appTarget":"shapier://conseils/recuperation-systeme-nerveux"},{"id":"b31","type":"quiz","title":"Check my understanding","questions":[{"id":"q1","prompt":"In which direction does information travel in the nervous system?","choices":[{"id":"q1c1","label":"Both ways at once, with an incoming flow larger than the outgoing one","correct":true,"explanation":"The page corrects the common depiction: the system does not run from the brain to the body, it works in both directions simultaneously."},{"id":"q1c2","label":"From the brain to the body, which carries out the commands","correct":false,"explanation":"This is the common depiction, but the flow rising from the body to the centre is even larger than the descending one."},{"id":"q1c3","label":"From the body to the brain, which then decides everything","correct":false,"explanation":"Motor commands also leave the centre for the body; both directions work at the same time."}]},{"id":"q2","prompt":"Does the autonomic nervous system work like a switch between its two sides?","choices":[{"id":"q2c1","label":"No, it is a continuous setting in which both are permanently active to varying degrees","correct":true,"explanation":"This is the nuance that popular accounts erase and that the page restores."},{"id":"q2c2","label":"Yes, one turns on while the other turns off","correct":false,"explanation":"This is the common presentation, but the page corrects it: the two sides stay active at the same time."},{"id":"q2c3","label":"Yes, and a simple technique can activate the side you choose","correct":false,"explanation":"The page states that such claims go well beyond what physiology establishes."}]},{"id":"q3","prompt":"Why does a reflex fire before you become aware of what is happening?","choices":[{"id":"q3c1","label":"Because the information enters the spinal cord and leaves it again without climbing to the brain, which is informed afterwards","correct":true,"explanation":"The route is shorter: it is a difference in circuit length, not a figure of speech."},{"id":"q3c2","label":"Because nerve signals travel instantly through the nerves","correct":false,"explanation":"A signal takes time to travel along a nerve, and that time depends on the calibre of the fibre."},{"id":"q3c3","label":"Because the brain decides faster in dangerous situations","correct":false,"explanation":"The reflex circuit does not climb to the brain at all; it is only informed afterwards."}]}]},{"id":"b32","type":"relatedContent","title":"Read next","targetIds":["en-mechanism-neuron","en-mechanism-motor-command","en-mechanism-neuromuscular-junction"]}],"relations":[{"type":"explains","targetId":"en-mechanism-motor-command"},{"type":"next-step","targetId":"en-mechanism-neuron"},{"type":"uses-structure","targetId":"en-anatomy-lumbar-spine"},{"type":"explains","targetId":"en-mechanism-aerobic-system"}],"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":"standring2020","authors":"Standring S","year":2020,"title":"Gray's Anatomy: The Anatomical Basis of Clinical Practice, 42nd edition","source":"Elsevier","kind":"book","url":"https://shop.elsevier.com/books/grays-anatomy/standring/978-0-7020-7705-0"}],"review":{"publishedAt":"2026-08-02","reviewedAt":"2026-08-02","reviewDueAt":"2027-08-02","evidenceLevel":"established"},"limitations":["No scene accompanies this page: the anatomical sources Body Lab uses supply only the skeleton, the muscles, the joints and the regions of the body. No nerve exists there as a mesh.","This page describes a general organisation. Exactly where functions sit in the nervous system is a more debated subject than common depictions suggest.","The conduction speeds cited are orders of magnitude measured on fibres of different calibres; they do not characterise any particular nerve.","No neurological assessment is possible from a page. Numbness, loss of strength, loss of sensation or any sudden disturbance belongs to a health professional, without delay."],"seo":{"title":"The nervous system — how the body gathers, decides and commands","description":"Central and peripheral, sensory and motor, voluntary and autonomic: the organisation of the network that commands movement and sets what is not decided.","canonicalPath":"/en/body/systems/the-nervous-system","image":"/og/en/mechanism-nervous-system.png"},"app":{"offlineEligible":true},"version":1}