A threshold stimulus is the resting potential. hyperpolarization of an axon. electrical current that crosses the synaptic cleft. depolarization necessary to cause an action potential. peak of an action potential.

Depolarization necessary to cause an action potential.

Explanation:

To achieve an action potential, you need to change the resting membrane potential from about -70 mV to +30 mV. This occurs by the influx of Na ions.

Now in order for this to happen, Na ions diffuse down electrochemical gradient to make the inside of the cell less negative. This effect is achieved by the opening of some Na+ voltage-gated channels.

To achieve the change from -70 mV to +30 mV, all the sodium voltage gated channels will need to open. This happens if the stimulus is strong enough to reach a potential of about -55 mV (threshold stimulus).

The concept behind this is positive feedback. The more depolarization there is, the more is the number of opened Na voltage gated channels.