Mechanism
Beginner
9 min read
The nervous system
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.
The system the corpus named without describing
Twenty-six pages of Body Lab mention a nerve. The motor command 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.
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.
Key point
The answer in three sentences
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.
A whole part of it works without the will, and that is indispensable: nobody could think about every heartbeat.
Two sets, one continuity
Central nervous system
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.
CNS
Peripheral nervous system
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.
PNS
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.
The spinal cord 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.
Two directions, at the same time
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.
What travels, and in which direction
Pathway
Direction
What it carries
Example
Sensory
From the body to the centre
Touch, position, tension, temperature
Feeling a foot slip
Motor
From the centre to the body
Commands to contract
Catching the slip
Autonomic
Both ways
Non-voluntary settings
Speeding the heart under effort
The sensory pathway includes a channel ordinary life never notices: proprioception, 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.
What does not go through a decision
Autonomic nervous system
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.
vegetative system
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.
That division illuminates several pages in this corpus. When the aerobic system page describes breathing and cardiac output rising at the start of an effort, this is the system acting, and nobody commands it.
Caution
A nuance popular accounts erase
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.
Claims that a simple technique can "activate" one side go well beyond what physiology establishes.
Fast, but not instant
A nerve signal takes time to travel along a nerve, and that time depends on the calibre of the fibre.
Certainty level · Established
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.
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.
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)
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.
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.
Why no scene accompanies this page
Limit
A system absent from the models
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.
This page therefore joins fascia, cartilage and dietary fat: 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.
The Body 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.
What this page does not do
Caution
No assessment, no diagnosis
Body Lab describes general mechanisms. This page allows no symptom to be interpreted and assesses no individual situation.
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.
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.
- Standring S (2020). Gray's Anatomy: The Anatomical Basis of Clinical Practice, 42nd edition. Elsevier.
Put it into practice in Shapier
Track your nervous system recovery
Tracking nervous fatigue and adjusting recovery between sessions happens in Shapier; Body Lab only explains how this system gathers, decides and commands.
Track your nervous system recoveryBody Lab explains; Shapier lets you act and track.
Check my understanding
In which direction does information travel in the nervous system?
Both ways at once, with an incoming flow larger than the outgoing one
From the brain to the body, which carries out the commands
From the body to the brain, which then decides everything
Does the autonomic nervous system work like a switch between its two sides?
No, it is a continuous setting in which both are permanently active to varying degrees
Yes, one turns on while the other turns off
Yes, and a simple technique can activate the side you choose
Why does a reflex fire before you become aware of what is happening?
Because the information enters the spinal cord and leaves it again without climbing to the brain, which is informed afterwards
Because nerve signals travel instantly through the nerves
Because the brain decides faster in dangerous situations
Choose an answer
Read next
- The neuron and the nerve signalA 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.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
- 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
Available offline
Offline download is available in the mobile app.
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: 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.
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.
- Standring S (2020). Gray's Anatomy: The Anatomical Basis of Clinical Practice, 42nd edition. Elsevier.
Educational content. Body Lab does not diagnose and does not replace professional advice.
How we work