What actually happens when a nerve cell fires?
Every thought and twitch rides on a tiny electrical spike that lasts a thousandth of a second. We learned how it works from a squid.
▶ Start the storyWhen a nerve cell fires, its voltage flips for about a thousandth of a second, and that flip races down its long fibre like a falling row of dominoes. At rest, the inside of a neuron sits at about −70 millivolts compared with the outside. Signals from other neurons nudge that number up and down. If it climbs to around −55 millivolts, tiny gates in the membrane called sodium channels snap open, sodium rushes in, the inside turns positive, and that opens even more gates. Then the sodium gates shut, potassium flows out, and the cell resets.
Step 1: Resting at −70 mV
The inside is negative compared with the outside
Step 2: Inputs reach −55 mV
Enough small nudges add up to the threshold
Step 3: Sodium rushes in
Open channels open more channels: the inside turns positive
Step 4: Potassium flows out
Sodium gates lock and the voltage drops back
Step 5: Brief recovery
The patch can't fire again at once, so the wave moves forward
Each spike triggers the same thing in the patch of membrane next door, so the signal is rebuilt at every step and arrives as strong as it started. The patch that just fired needs a moment to recover, which is why the wave only moves forward. And a spike is all or nothing: a harder pinch doesn't make a bigger spike, it makes more of them per second.
We know this thanks to a squid. Its escape reflex runs on a giant nerve fibre about a millimetre wide, a hundred times a typical neuron and visible to the naked eye. Alan Hodgkin and Andrew Huxley slid wires inside it and worked out the whole sequence in 1952, earning a Nobel Prize in 1963. Your own nerves found a cleverer trick than sheer size: a fatty wrapping called myelin that makes the signal hop, so thin fibres can carry it at over 100 metres per second.
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Recap
Sodium in, potassium out, rebuilt at every step: a spike never gets bigger, only more frequent.
Surprising fact · Hodgkin and Huxley cracked the mechanism in 1952 using the squid's giant axon, about 100 times wider than a typical neuron.
Sources (2)
No source, no claim. Every fact in this lesson (21 claims) cites at least one of these.