How Neurons Work: The Brain to Muscle Journey
A signal leaves your brain, runs down your spinal cord, along a nerve in your arm and into a muscle, and the whole trip takes about a fiftieth of a second. Most people picture that as electricity flowing down a wire. It is not, and the difference is the most interesting thing about nerves: nothing travels the length of the cell. A wave of tiny gates opening does, one after another, like a stadium crowd standing up in turn.
Everything below comes from the 3Eyes Brain & Neurons lesson - a 3D interactive where a child zooms into one neuron, labels its parts and sends a signal from a real brain model down a nerve to a muscle. The recordings below are taken straight from it; the lesson itself is part of 3Eyes.
This is the neuron from the outside in - its parts, the route a signal takes, and what is actually moving.
A neuron has five parts worth knowing
Every neuron is the same basic shape, stretched to fit its job.
The cell body holds the nucleus and does the housekeeping, like any other cell. Branching off it are the dendrites - short, tree-like arms that collect signals coming in from other neurons. The axon is the long cable that carries the neuron's own signal outward; in a nerve running from your spine to your toe, a single axon can be a metre long. Along the axon, fatty myelin is wrapped in segments with small gaps between them, and at the far end the axon splits into terminals that pass the signal on.
Where a terminal meets the next cell there is a gap called the synapse. The signal has to cross it, and it does so chemically, which is the reason medicines and drugs can change how you feel.
The signal is a wave of gates, not a current
The membrane of a neuron is studded with tiny doors that let charged atoms - ions - through. At rest the neuron pumps sodium ions out and keeps the inside slightly negative. When a signal arrives at the start of the axon, sodium doors there open, sodium rushes in, and that patch of membrane flips positive. The flip opens the doors just next to it, which flip, which open the next ones.
So what travels is the flip, not the ions. Each patch of membrane does a little work and then resets, pumping the sodium back out and letting potassium ions through to restore the balance. Nothing streams from your brain to your hand. The stadium wave is the right picture, and it is why the signal never fades along the way: every patch regenerates it at full strength.
Why the myelin gaps make it fast
Bare axons are slow - around a metre per second. That would put a full second between deciding to move your foot and the foot moving, which is not survivable.
Myelin fixes it. The fatty wrapping insulates the axon so the flip cannot happen underneath it; it can only happen at the gaps, called nodes of Ranvier. The signal therefore jumps from gap to gap instead of crawling along every millimetre, and jumping is about a hundred times faster - up to 120 metres per second in the thickest nerves. You can see it in the video above as the glow skipping between nodes.
This is what multiple sclerosis damages: the myelin, not the neuron. The cell survives but its signals slow down and stumble, which is why the symptoms are about speed and coordination.
From brain to muscle in three neurons
The lesson lets you follow one signal from a real brain model to an arm muscle, and the route is shorter than most children expect.
- A motor neuron in the brain fires. Its axon runs down through the brainstem into the spinal cord.
- At the spinal cord it synapses with a second motor neuron, whose axon leaves the spine as part of a nerve.
- That nerve reaches the muscle, and the axon terminals release a chemical - acetylcholine - across the synapse onto the muscle fibre, which contracts.
Two synapses, two cells, one muscle. A reflex - pulling your hand off something hot - is shorter still: the sensory signal reaches the spinal cord and is routed straight back out to the muscle without going up to the brain at all, which is why your hand moves before you know why.
The numbers
| Value | Why it matters | |
|---|---|---|
| Neurons in the brain | ~86 billion | Each connects to thousands of others |
| Signal speed, bare axon | ~1 m/s | Too slow for a body of any size |
| Signal speed, myelinated | up to 120 m/s | The gaps let the signal jump |
| Time to cross a synapse | ~1 millisecond | The chemical hop is the slow step |
| Longest axon in the body | ~1 metre | From the base of the spine to the toes |
| Brain to hand | ~20 milliseconds | Faster than a blink |
How to explain it in five minutes
Start with the stadium wave. Nobody runs round the stadium; everyone stands up in turn and the wave crosses it. That is a nerve signal, and it explains at once why it never fades.
Then the parts, as a journey. Dendrites collect, cell body decides, axon carries, terminals hand over. Four parts and one verb each.
Then the gaps. Ask why the myelin has breaks in it. The answer - so the signal can jump - is the fact children repeat to other people.
Finish with the reflex. Hot pan, hand moves, then you feel it. The order surprises everyone, and it is the proof that the spinal cord makes decisions too.
Where 3Eyes Fits
The neuron lesson above is one of about sixty interactive science lessons built into 3Eyes, which is a parental controls product for family computers - it filters the machine, approves sites rather than blocking them one at a time, and limits YouTube to channels you choose.
The lessons are there because a computer that only ever says no is a difficult thing to defend to a ten-year-old. When the same laptop has sixty science lessons and a tutor on it, the limits stop being the entire relationship.
About two minutes to set up. See what it includes or create an account.
Frequently asked questions
What are the main parts of a neuron? The cell body (with the nucleus), dendrites that receive signals, the axon that carries the neuron's own signal, the myelin sheath insulating the axon, and the axon terminals that pass the signal across a synapse to the next cell.
How does a signal travel through a neuron? As a wave of ion gates opening along the membrane: sodium rushes into one patch, which triggers the next patch, and so on down the axon. Each patch resets behind the wave, so the signal is regenerated at full strength the whole way.
Is a nerve signal electricity? It is electrical - charges are moving - but it is not a current flowing down a wire. Ions cross the membrane sideways; nothing travels the length of the nerve except the wave of activity itself. That is why it is a hundred thousand times slower than electricity in a cable.
What does myelin do? It insulates the axon in segments so the signal can only regenerate at the gaps between them. Jumping gap to gap is far faster than travelling every millimetre, which is why myelinated nerves carry signals at up to 120 m/s.
How fast do nerve signals travel? Between about 1 and 120 metres per second depending on the nerve. Brain to hand takes roughly 20 milliseconds; a spinal reflex is quicker still because it skips the brain.