{"schemaVersion":1,"id":"en-mechanism-neuron","type":"mechanism","pillar":"body","locale":"en","translationGroupId":"mechanism-neuron","slugPath":"body/systems/the-neuron","title":"The neuron and the nerve signal","shortTitle":"The neuron","summary":"A cell that always sends the same signal, no stronger and no weaker. So it is not the size of the message that codes the intensity of an effort, but its rate.","level":"intermediate","estimatedMinutes":9,"synonyms":["neuron","nerve impulse","action potential","axon","synapse","myelin","neurotransmitter"],"blocks":[{"id":"b1","type":"heading","level":2,"text":"A cell of an unusual shape","anchor":"a-cell-of-an-unusual-shape"},{"id":"b2","type":"paragraph","content":[{"kind":"text","value":"Most cells in the body are compact. The neuron is not: it is built to link two distant places, and its whole shape follows from that function."}]},{"id":"b3","type":"paragraph","content":[{"kind":"text","value":"Three parts make it up. The "},{"kind":"strong","value":"dendrites"},{"kind":"text","value":" are branches receiving signals from other cells. The "},{"kind":"strong","value":"cell body"},{"kind":"text","value":" gathers them. The "},{"kind":"strong","value":"axon"},{"kind":"text","value":" is a single extension leaving that body and carrying the signal to its destination."}]},{"id":"b4","type":"paragraph","content":[{"kind":"text","value":"The length of the axon is what stands out most. The motor neuron commanding a muscle of the foot has its cell body in the spinal cord, in the low back, and its axon runs down to the leg: more than a metre, within a single cell."}]},{"id":"b5","type":"definition","term":"Axon","definition":"The single elongated extension of a neuron, by which the signal leaves the cell. It is what we call a nerve fibre, and a nerve bundles thousands of them.","alsoKnownAs":["nerve fibre"]},{"id":"b6","type":"heading","level":2,"text":"The signal does not vary","anchor":"the-signal-does-not-vary"},{"id":"b7","type":"paragraph","content":[{"kind":"text","value":"This is the most important point on the page, and the most counter-intuitive."}]},{"id":"b8","type":"evidence","level":"established","statement":"The signal travelling along an axon — the action potential — obeys an all-or-nothing rule: it fires beyond a threshold, and its amplitude does not depend on the intensity of what triggered it.","detail":"This is a founding result of neuroscience, obtained by direct recording on isolated fibres and since reproduced across every model. A stronger stimulus does not produce a bigger signal: it produces more of them per second.","referenceIds":["kandel2021","purves2018"]},{"id":"b9","type":"paragraph","content":[{"kind":"text","value":"The consequence is decisive for understanding movement. If the signal is always identical, the intensity of an effort cannot be coded by its size. It is coded by its "},{"kind":"strong","value":"rate"},{"kind":"text","value":": the higher the demand, the faster the impulses follow one another."}]},{"id":"b10","type":"paragraph","content":[{"kind":"text","value":"This is exactly the second lever the "},{"kind":"link","value":"motor command","href":"/en/adaptations/motor-command","external":false},{"kind":"text","value":" page describes, alongside recruiting further units. The body has two settings: how many units it activates, and how often it calls on them."}]},{"id":"b11","type":"comparison","title":"Two ways to produce more force","columns":["Lever","What changes","Analogy"],"rows":[{"label":"Recruitment","cells":["The number of active neurons","Opening more taps"]},{"label":"Firing rate","cells":["The number of impulses per second","Opening the same ones more often"]}]},{"id":"b12","type":"heading","level":2,"text":"Why some signals travel fast","anchor":"why-some-signals-travel-fast"},{"id":"b13","type":"definition","term":"Myelin","definition":"A fatty wrapping surrounding some axons in segments, interrupted at regular intervals. It insulates the fibre electrically and markedly speeds up propagation of the signal.","alsoKnownAs":["myelin sheath"]},{"id":"b14","type":"paragraph","content":[{"kind":"text","value":"On a bare fibre the signal spreads step by step, which is slow. On a sheathed fibre it regenerates only at the interruptions of the sheath and jumps from one to the next. The distance covered per unit of time rises by an order of magnitude."}]},{"id":"b15","type":"paragraph","content":[{"kind":"text","value":"This is what explains the speed gap described on the "},{"kind":"link","value":"nervous system","href":"/en/body/systems/the-nervous-system","external":false},{"kind":"text","value":" page: the fibres carrying muscle commands and position information are thick and well sheathed, hence fast. Those carrying some slower information are not."}]},{"id":"b16","type":"heading","level":2,"text":"Passing from one cell to another","anchor":"passing-from-one-cell-to-another"},{"id":"b17","type":"paragraph","content":[{"kind":"text","value":"Two neurons do not touch. A gap remains between them, and the electrical signal cannot cross it."}]},{"id":"b18","type":"definition","term":"Synapse","definition":"The contact zone between two nerve cells, where the electrical signal is converted into a chemical one to cross the gap separating them, then converted back into an electrical signal.","alsoKnownAs":["junction between two neurons"]},{"id":"b19","type":"paragraph","content":[{"kind":"text","value":"The detour looks pointless. It is not: this is precisely where the system gains its flexibility. A synapse can strengthen a signal, weaken it, block it entirely, or lastingly change its own effectiveness. A purely electrical cable would allow none of that."}]},{"id":"b20","type":"paragraph","content":[{"kind":"text","value":"The molecules crossing the gap are called neurotransmitters. There are dozens of them, and their effect depends as much on the receptor receiving them as on the molecule itself."}]},{"id":"b21","type":"callout","tone":"caution","title":"What common accounts make neurotransmitters say","content":[[{"kind":"text","value":"\"Dopamine, the pleasure molecule.\" \"Serotonin, the happiness hormone.\" These formulas are very widespread and do not match what the literature describes."}],[{"kind":"text","value":"The same molecule acts differently depending on the region, the receptor and the context. A neurotransmitter is not the chemical equivalent of an emotion, and no simple correspondence between a molecule and a mental state is established."}],[{"kind":"text","value":"Body Lab does not write those shortcuts, and addresses no question of mental health: that would be outside its field."}]]},{"id":"b22","type":"heading","level":2,"text":"What this page simplifies","anchor":"what-this-page-simplifies"},{"id":"b23","type":"callout","tone":"limit","title":"A step made cleaner than it is","content":[[{"kind":"text","value":"The all-or-nothing rule describes what travels along the axon. What happens before is different: signals arriving on the dendrites and the cell body add and subtract in a graded way, and it is their sum that does or does not cross the threshold."}],[{"kind":"text","value":"In other words, the decision is graded, the outcome is binary. This page describes mostly the second half."}],[{"kind":"text","value":"No scene accompanies it, for two cumulative reasons: the anatomical sources used contain no nervous tissue, and a cell in any case sits far below what an anatomical mesh represents."}]]},{"id":"b24","type":"heading","level":2,"text":"Sources","anchor":"sources"},{"id":"b25","type":"sourceList","title":"Main sources","referenceIds":["kandel2021","purves2018"]},{"id":"b26","type":"shapierAction","actionId":"conseils-recuperation-nerveux","label":"Planning nervous-system recovery","description":"Planning nervous-system recovery between sessions happens in Shapier; Body Lab only explains how the nerve signal is transmitted.","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":"When muscular demand rises, how does the nervous system code that intensity?","choices":[{"id":"q1c1","label":"By increasing the firing rate of the impulses","correct":true,"explanation":"The action potential is always identical; it is the number of impulses per second that rises with demand."},{"id":"q1c2","label":"By increasing the amplitude of the signal","correct":false,"explanation":"A plausible idea, since stronger seems to mean bigger, but the all-or-nothing rule forbids any variation in amplitude."},{"id":"q1c3","label":"By lengthening each action potential","correct":false,"explanation":"The page describes no change in duration; only the frequency of the impulses changes."}]},{"id":"q2","prompt":"What should we make of the common formula dopamine, the pleasure molecule?","choices":[{"id":"q2c1","label":"It does not match what the literature describes","correct":true,"explanation":"The same molecule acts differently depending on the region, the receptor and the context; no simple correspondence between a molecule and an emotion is established."},{"id":"q2c2","label":"It is accurate but incomplete","correct":false,"explanation":"This is the widespread wording, but the page states it does not match the literature; it is not a simplified version of an accurate fact."},{"id":"q2c3","label":"It holds for dopamine but not for serotonin","correct":false,"explanation":"The page treats both formulas the same way: no simple correspondence is established, whatever the molecule."}]},{"id":"q3","prompt":"Does the all-or-nothing rule apply to the whole neuron?","choices":[{"id":"q3c1","label":"No, it describes the signal travelling along the axon","correct":true,"explanation":"Upstream, the signals arriving on the dendrites and the cell body add and subtract in a graded way."},{"id":"q3c2","label":"Yes, from the dendrites all the way to the end of the axon","correct":false,"explanation":"That is what the rule would suggest, but the page qualifies it: the decision is graded, only the outcome is binary."},{"id":"q3c3","label":"No, it applies only at the synapses","correct":false,"explanation":"The synapse converts the signal into a chemical one, but that is not where the all-or-nothing rule described by the page applies."}]}]},{"id":"b28","type":"relatedContent","title":"Read next","targetIds":["en-mechanism-neuromuscular-junction","en-mechanism-nervous-system","en-mechanism-motor-command"]}],"relations":[{"type":"prerequisite","targetId":"en-mechanism-nervous-system"},{"type":"explains","targetId":"en-mechanism-motor-command"},{"type":"next-step","targetId":"en-mechanism-neuromuscular-junction"}],"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"}],"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, and a cell this size would in any case sit far below what an anatomical mesh represents.","The all-or-nothing rule describes the signal travelling along the axon. What happens upstream, as signals arrive on the cell, is graded: the page simplifies that step.","The role of a neurotransmitter does not reduce to one function or one emotion. Simple correspondences between a molecule and a mental state have no basis in the literature cited.","This page addresses no question of mental health or neurology, and allows no symptom to be interpreted."],"seo":{"title":"The neuron — why it is the rate of the signal that counts","description":"Axon, action potential, myelin and synapse: how a cell that always sends the same signal manages to code an intensity.","canonicalPath":"/en/body/systems/the-neuron","image":"/og/en/mechanism-neuron.png"},"app":{"offlineEligible":true},"version":1}