From Threshold to Spike
If enough excitatory input pushes a neuron from its resting potential ( mV) up to about mV, it crosses threshold and fires an action potential: a rapid voltage spike peaking at mV.
The all-or-none law states that once threshold is crossed, the spike always reaches the same peak regardless of how much threshold was exceeded. A sub-threshold stimulus produces nothing.
| Phase | What happens |
|---|---|
| Depolarization | Voltage-gated Na+ channels open; Na+ rushes in, driving voltage toward mV |
| Repolarization | Na+ channels inactivate; voltage-gated K+ channels open, K+ rushes out |
| Hyperpolarization | K+ channels close slowly, briefly overshooting below mV |
| Return to rest | The Na+/K+ pump and leak channels restore resting potential |
Refractory Periods & Speed
During the absolute refractory period (when Na+ channels are inactivated), no stimulus can trigger a second spike, enforcing one-way propagation. During the relative refractory period, only an unusually strong stimulus can fire a spike.
Myelin speeds conduction dramatically. The impulse jumps between Nodes of Ranvier via saltatory conduction, reaching up to 120 m/s compared to 1 m/s in unmyelinated axons.
Common pitfall: Thinking a stronger stimulus produces a bigger action potential. Intensity is coded by firing rate, not spike size.