Final answer:
When Na+ ions enter the axon, their movement is initially bidirectional, but the progression of the action potential toward the synapse is more pronounced. The refractory period behind the action potential prevents backward movement, making the forward propagation more obvious.
Step-by-step explanation:
When Na+ ions enter an axon through voltage-sensitive sodium channels during an action potential, they do so due to the electrochemical gradient. The movement of Na+ ions is bidirectional initially, but the directional flow down the axon toward the synapse is more prominent. This is because the depolarization spreads sequentially along the axon, and the region behind the action potential undergoes a refractory period, which prevents backward movement. The propagation of the action potential is facilitated by saltatory conduction in myelinated axons, where it appears to 'jump' from one node of Ranvier to the next.