Final answer:
An action potential will depolarize the membrane only when the membrane potential reaches the threshold of excitation.
Step-by-step explanation:
An action potential is an electrical signal that travels down a neuron's axon. It follows the 'all-or-none' law, which means that it will depolarize the membrane only when the membrane potential reaches the threshold of excitation.
Once the threshold is reached, the action potential will propagate all the way down the axon without diminishing in strength.