Why does a neuron either fire a full spike or none at all? · Grey Matter

Voltage-gated sodium channels open when the membrane depolarises, and the sodium they let in depolarises it further, which opens more of them.

Why does a neuron either fire a full spike or none at all?

Voltage-gated sodium channels open when the membrane depolarises, and the sodium they let in depolarises it further, which opens more of them. Below threshold, potassium leaking out wins and the voltage drifts back; once enough sodium channels open that the inward current outruns the outward one, the feedback runs away and the membrane swings to its peak. The size of the spike is then set by the sodium gradient and the number of channels, never by the size of the input that crossed threshold. This positive feedback was first described quantitatively by Hodgkin and Huxley in the squid giant axon.