Intro | Depolarization | Hyperpolarization | Action Potential | Threshold of Excitation
Part 1: Image-Mapped Tutorial
Part 2: Matching Self-Test
Part 3: Multiple-Choice Self-Test
The all-or-none response, or Action Potential, occurs only when the threshold of excitation has been exceeded and it is of consistent size (peak depolarization of +40 to +50 mV) and shape (duration of transition less than 1 millisecond). It is a massive, local, and brief reversal of polarity across the axon membrane under stimulation. The voltage dependent response of the sodium channels supports the rapid rising phase of an action potential. Each open sodium channel can allow the influx of up to 100 million ions per second. It is important to note, however, that this massive but local influx of ions at the point of depolarization typically raises the concentration of sodium within the neuron overall by less than 1 percent.