Final answer:
When an electrode triggers depolarization above the threshold in the middle of an axon, action potentials will start at that point and travel in both directions due to the all-or-nothing nature of action potentials and the refractory period that follows depolarization.
Step-by-step explanation:
If an electrode is placed in the middle of an axon at resting membrane potential, and an above-threshold depolarization is applied, action potentials will start at that point and travel in both directions in the axon. This occurs because the action potential is an all-or-nothing event: once the threshold of excitation is reached, the action potential triggers and depolarization occurs, causing the voltage-gated Na+ channels to open. As a result, Na+ ions rush into the axon, reversing the polarity from the negative resting potential to a positive potential, leading to further depolarization along the axon in both directions.
Depolarization proceeds in a wave down the length of the axon, and since the sodium channels are inactivated after opening (preventing an immediate subsequent action potential), the action potential can only travel away from the recently depolarized area. This prevents the reverse propagation of the action potential back over the same patch of membrane, ensuring unidirectional propagation along each segment of the axon.
This all-or-nothing principle and the refractory period ensure that action potentials propagate in both directions from the point of stimulation but do not re-stimulate areas that have just been activated.