Mechanism
Intermediate
9 min read
The neuron and the nerve signal
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.
A cell of an unusual shape
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.
Three parts make it up. The dendrites are branches receiving signals from other cells. The cell body gathers them. The axon is a single extension leaving that body and carrying the signal to its destination.
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.
Axon
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.
nerve fibre
The signal does not vary
This is the most important point on the page, and the most counter-intuitive.
Certainty level · Established
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.
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.
Kandel ER, Koester JD, Mack SH, Siegelbaum SA (2021) · Purves D, Augustine GJ, Fitzpatrick D, Hall WC, LaMantia AS, Mooney RD, Platt ML, White LE (2018)
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 rate: the higher the demand, the faster the impulses follow one another.
This is exactly the second lever the motor command page describes, alongside recruiting further units. The body has two settings: how many units it activates, and how often it calls on them.
Two ways to produce more force
Lever
What changes
Analogy
Recruitment
The number of active neurons
Opening more taps
Firing rate
The number of impulses per second
Opening the same ones more often
Why some signals travel fast
Myelin
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.
myelin sheath
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.
This is what explains the speed gap described on the nervous system page: the fibres carrying muscle commands and position information are thick and well sheathed, hence fast. Those carrying some slower information are not.
Passing from one cell to another
Two neurons do not touch. A gap remains between them, and the electrical signal cannot cross it.
Synapse
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.
junction between two neurons
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.
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.
Caution
What common accounts make neurotransmitters say
"Dopamine, the pleasure molecule." "Serotonin, the happiness hormone." These formulas are very widespread and do not match what the literature describes.
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.
Body Lab does not write those shortcuts, and addresses no question of mental health: that would be outside its field.
What this page simplifies
Limit
A step made cleaner than it is
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.
In other words, the decision is graded, the outcome is binary. This page describes mostly the second half.
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.
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.
Put it into practice in Shapier
Planning nervous-system recovery
Planning nervous-system recovery between sessions happens in Shapier; Body Lab only explains how the nerve signal is transmitted.
Planning nervous-system recoveryBody Lab explains; Shapier lets you act and track.
Check my understanding
When muscular demand rises, how does the nervous system code that intensity?
By increasing the firing rate of the impulses
By increasing the amplitude of the signal
By lengthening each action potential
What should we make of the common formula dopamine, the pleasure molecule?
It does not match what the literature describes
It is accurate but incomplete
It holds for dopamine but not for serotonin
Does the all-or-nothing rule apply to the whole neuron?
No, it describes the signal travelling along the axon
Yes, from the dendrites all the way to the end of the axon
No, it applies only at the synapses
Choose an answer
Read next
- The neuromuscular junctionThe place where a nerve command becomes a contraction. This corpus described both sides of that crossing without ever describing the crossing itself — and it holds a surprise: relaxing costs energy.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
- Motor commandA muscle does not contract as one block. It contracts in units, recruited in an order that is not left to chance — and that recruitment improves before the muscle gets any bigger.With a 3D 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, 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.
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.
Educational content. Body Lab does not diagnose and does not replace professional advice.
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