Final answer:
When an electrode induces an above-threshold voltage in a resting axon, action potentials start at that point and travel in both directions. Action potentials involve all-or-none responses through depolarization and repolarization phases, enabled by the opening and closing of voltage-gated ion channels.
Step-by-step explanation:
Action Potentials in Axons
When an electrode is placed in the middle of a resting axon and an above-threshold voltage is applied, action potentials will start at that point and travel in both directions in the axon. The action potential is an all-or-none event, meaning that it will either occur fully or not at all once the threshold of excitation is reached. Once initiated, the action potential travels down the axon through the opening of voltage-gated ion channels. These channels allow for the rapid influx of Na+ ions (depolarization) followed by the efflux of K+ ions (repolarization), propagating the action potential without decrement. After an action potential has occurred, a refractory period prevents another action potential from being initiated immediately at the same spot. Because of the arrangement of voltage-gated Na+ channels, the action potential does not move in one direction only; rather, it propagates away from the point of initiation in both directions. This is due to the fact that once a segment of the axon is depolarized, it serves as a stimulus for neighboring regions of the axon membrane, leading to a chain reaction that propagates the action potential. After the action potential has passed, the Na+/K+ pump helps to restore the resting membrane potential, preparing the axon for the next action potential.