The All-or-None Spike

A light switch, not a dimmer

Picture a light switch on the wall. You can flick it gently or slam it hard, but the light comes on exactly the same either way. A neuron's signal works like that switch. Once the cell is pushed past a certain tipping point, it fires a full electrical pulse called an action potential — and it is always the same size, no matter how hard it was pushed. This is the all-or-none principle: you get the whole spike, or you get nothing.

The tipping point has a name: the threshold. Small nudges that don't reach it fade away quietly. But the instant a nudge crosses threshold, the switch flips and the spike erupts on its own. Nothing in between — no half-spike for a half-push.

Up the mountain: depolarization to the peak

At rest, the inside of the neuron sits slightly negative compared to the outside — its resting membrane potential, around −70 millivolts. Think of it as a coiled spring, held tense and ready. When a nudge crosses threshold, tiny gates called voltage-gated channels swing open and positive sodium ions pour inward.

That inrush of positives makes the inside rapidly less negative, then positive — a swing called depolarization. And here is the clever part: each bit of depolarization opens more sodium gates, which lets in more positives, which opens still more gates. The signal feeds itself, rocketing up to a sharp peak near +30 mV. That self-driving surge is exactly why the spike is always full-sized.

  +30 mV ─┐ peak
          │\
          │ \  repolarization
  depol.  │  \
         /│   \
   ─────/ │    \____ resting (−70 mV)
 threshold      \  /
                 \/ undershoot

Down again, then a forced pause

A spike that only went up would be a stuck switch. So at the peak, the sodium gates snap shut and a second set of gates lets positive potassium ions flow out. Losing those positives drags the inside back toward negative — this is repolarization, the downhill half of the spike. The cell often dips a little past rest before settling, like a swing overshooting at the bottom.

Right after a spike comes the refractory period — a short stretch when the neuron either cannot fire at all, or can only fire if pushed extra hard. The sodium gates need a moment to reset, like a camera flash recharging before it can pop again. This pause is small but mighty: it does two crucial jobs.

1. It keeps signals moving one way. The patch just behind a spike is still resetting, so the wave can only push forward — never backward into where it came from.
2. It spaces spikes out, capping how fast a neuron can fire. No matter how strong the input, spikes can't pile on top of each other.

If every spike is identical, where's the message?

Here's the puzzle the all-or-none rule hands us: if a brighter light or a louder sound can't make a spike any bigger, how does the neuron say "this is strong"? The answer is that strength can't ride on the size of a spike — so it rides on how many and when. A faint touch might trigger a few spikes per second; a firm press, a rapid burst.

Leaping down a wire: saltatory conduction

Once a spike is born, it has to travel — sometimes a long way down the neuron's wire. In many neurons that wire is wrapped in a fatty insulating sleeve, with bare gaps left at regular intervals. The spike doesn't crawl along every inch; instead it jumps from gap to gap, regenerating itself fresh at each one. This leaping is called saltatory conduction, and it makes signals fly far faster than they otherwise would.

Crucially, because every spike is all-or-none, each leap rebuilds the pulse to full size. The signal never weakens with distance the way a shout fades down a hallway — it arrives at the far end just as sharp as it began. Same switch, flipped over and over, all the way to the destination.