Feynman Note — The Action Potential Explained Simply — Notes

Explaining the Action Potential Like You're Explaining to a 12-Year-Old

Imagine a neuron is like a water-pipe. At rest, it is sealed tightly. The "inside water" (K+) is trying to get out, and the "outside water" (Na+) is trying to get in, but both are blocked by the pipe walls (ion channels, which are closed).

Now imagine someone flicks the pipe at one end (a stimulus). This flick pushes open a "Na+ door" (voltage-gated Na+ channel). Suddenly Na+ rushes in from outside because:

There's MORE Na+ outside (like pressure from a full tank), and
The inside is negative (Na+ is positive, and opposites attract).

This surge of Na+ makes the inside of the pipe momentarily positive (+30 mV). This is the action potential spike.

But the body doesn't want to stay positive, so:

The "Na+ door" slams shut (Na+ channel inactivates).
A "K+ door" opens. K+ rushes out (there's more K+ inside, and now the inside is positive, so K+ is repelled out). This restores the negative inside (repolarization).

After the flick at the first point, the electric disturbance flows forward and flicks the NEXT section of the pipe, opening its Na+ doors. And so on — the signal "travels" along the neuron like a wave.

But there's a catch: the section that just fired can't fire again immediately (the Na+ doors are broken shut for a moment — refractory period). This ensures the wave only travels FORWARD and doesn't go backwards.

In myelinated neurons? The pipe is wrapped in waterproof insulation (myelin) except at small gaps (nodes of Ranvier). The signal skips from gap to gap — it doesn't have to flick every single centimetre. This is saltatory conduction — much faster, like a relay race where the baton jumps ahead.

What Does This Mean for Real Life?

When you touch a hot stove, the Na+ doors open first in your fingertip sensory neuron, then in the next neuron, then in the spinal cord, and within milliseconds your motor neuron fires and your hand pulls back — all before you're consciously aware of the pain (because pain travels via slow C fibres, but the withdrawal reflex runs via faster A-delta fibres).