At rest, the inside of a neuron sits at –70 mV — more negative than the outside. This is the resting membrane potential, maintained by an unequal distribution of ions across the cell membrane.

The key ions involved:

• Sodium (Na⁺) — concentrated outside the cell
• Potassium (K⁺) — concentrated inside the cell

When graded potentials push the membrane to –55 mV (threshold), voltage-gated sodium channels snap open:

1. Na⁺ rushes in → inside becomes positive (rises to about +30 mV)
2. K⁺ rushes out → positive charge leaves, inside goes negative again
3. Membrane returns to –70 mV resting potential

This rapid flip — from –70 to +30 and back — is the action potential. It takes about 2–4 milliseconds.

🚽 The toilet flush analogy: push the handle gently and nothing happens. Push it hard enough and the full flush always occurs — same amount of water every time, regardless of how firmly you pushed. Action potentials work the same way: once threshold is crossed, the spike is always the same size. This is the all-or-nothing principle.

After each spike, the neuron enters a refractory period — a brief window where it cannot fire again regardless of how strong the next input is. This is like the toilet tank refilling: you have to wait before the next flush is possible.

The refractory period sets the neuron's maximum firing frequency. Even under constant strong stimulation, there's a ceiling on how many spikes per second a neuron can produce. This is part of why there's a maximum speed at which you can wiggle your fingers — motor neurons can only signal so fast.