Final answer:
Action potentials are able to travel over long distances without decaying because of saltatory conduction in myelinated axons and the presence of voltage-gated channels at specific points called nodes of Ranvier.
Step-by-step explanation:
The action potential travels down the axon as voltage-gated ion channels are opened by the spreading depolarization. In unmyelinated axons, this happens in a continuous fashion because there are voltage-gated channels throughout the membrane. In myelinated axons, propagation is described as saltatory conduction because voltage-gated channels are only found at the nodes of Ranvier and the electrical events seem to "jump" from one node to the next.
Saltatory conduction is faster than continuous conduction, meaning that myelinated axons propagate their signals faster. The diameter of the axon also makes a difference as ions diffusing within the cell have less resistance in a wider space.
Because voltage-gated Na+ channels are inactivated at the peak of the depolarization, they cannot be opened again for a brief time-the absolute refractory period. Because of this, depolarization spreading back toward previously opened channels has no effect. The action potential must propagate toward the axon terminals; as a result, the polarity of the neuron is maintained.