How Your Brain Sends a Signal
Touch something hot and your hand is moving before you have decided anything. That gap - between the burn happening and you knowing about it - is the most interesting thing about nerves, and it comes down to how a single cell decides whether to fire.
A nerve cell is a body, a wire and an end
Three parts, and the names matter less than the shape.
There is a body, which collects incoming messages. There is a long thin wire running away from it, which can be anything from a fraction of a millimetre to nearly a metre in a leg. And there is an end, which passes the message to whatever comes next.
That is the whole design, repeated about eighty-six billion times. What makes it interesting is the rule the body uses to decide whether to send anything down the wire at all.
All or nothing
A nerve does not send a small signal for a small poke and a big signal for a big one. It either fires at full strength or it does not fire.
The body of the cell adds up everything arriving from its neighbours. If the total stays below a threshold, nothing happens - no partial signal, no weak version. Cross the threshold and it fires completely, at exactly the same size every time.
This is genuinely counter-intuitive, and it is very hard to convey in a sentence. It is obvious in about ten seconds of watching: nudge the cell gently and the voltage trace wobbles and settles back. Nudge it harder and the whole thing goes off.
Then how do you feel a difference?
If every signal is identical, how does a hard pinch feel different from a light one? Two ways, and neither involves a bigger signal.
More often. A strong stimulus makes the cell fire many times a second instead of a few. The size never changes; the rate does.
More cells. A strong stimulus recruits more nerve cells at once. One firing is a touch; hundreds firing together is pain.
So your brain reads intensity from frequency and headcount, never from amplitude. It is closer to Morse code than to a volume dial - which is a comparison children take to quickly.
The signal is not electricity in a wire
Worth separating from how electricity actually works, because the two get conflated constantly.
A nerve does not conduct electrons along a copper wire. It works by moving charged particles - mostly sodium - in and out through gates in its wall. Gates open, particles rush in, the voltage at that spot flips, which opens the gates just ahead, which flips the next spot. The signal is a wave of gates opening down the length of the cell, each one triggering the next.
That is why it is much slower than electricity in a wire, and why a nerve needs to actively pump those particles back afterwards to be ready to fire again.
Crossing the gap
Nerve cells do not touch. There is a tiny gap at the end of each one, and the electrical signal cannot jump it.
Instead the end of the cell releases chemicals into the gap. They drift across - a very short distance, but a real one - and are caught by receptors on the far side. If enough are caught, the next cell's body starts adding up towards its own threshold, and the whole process runs again.
This chemical step is the slow part of the chain, and it is also where nearly everything that affects the brain does its work. Caffeine, anaesthetics, antidepressants and most drugs act on what happens in these gaps rather than on the wires.
The numbers
| Roughly | |
|---|---|
| Nerve cells in a human brain | 86 billion |
| Fastest signal speed | about 120 metres per second |
| Slowest signal speed | about 1 metre per second |
| Width of the gap between cells | about 20 nanometres |
| Time for one firing | about 1-2 milliseconds |
The gap is the number worth pausing on. Twenty nanometres is about four thousand times thinner than a human hair, and every thought anyone has ever had has crossed billions of them chemically.
Where 3Eyes Fits
The neuron lesson above is one of about sixty interactive science lessons built into 3Eyes, a parental controls product for family computers - it filters the machine, approves sites rather than blocking them individually, and limits YouTube to channels you choose.
They exist because a computer that only refuses things is a difficult thing to defend to a child. With sixty science lessons and a tutor on the same laptop, the limits are no longer the entire relationship.
About two minutes to set up. See what it includes or create an account.
Frequently asked questions
What does all-or-nothing mean? A nerve cell either fires at full strength or does not fire at all. There is no small signal for a small stimulus. Below the threshold nothing happens; above it, the same full-size signal every time.
How does the brain know if something is a lot or a little? From how often cells fire and how many fire at once, never from the size of the signal. Frequency and headcount, not amplitude.
How fast is a nerve signal? Up to about 120 metres per second in the fastest nerves, and as slow as about 1 metre per second in others. Far slower than electricity in a wire, because it is a chemical and electrical process rather than electrons flowing.
Do nerve cells touch each other? No. There is a gap of about 20 nanometres, crossed by chemicals released from one side and caught by the other.
Why does your hand move before you feel the pain? The reflex is handled by a short loop through the spinal cord, which does not wait for the signal to reach your brain and come back. You move first and find out afterwards.