Final answer:
The action potential is propagated along the axon by the opening of voltage-gated Na+ channels, and saltatory conduction in myelinated axons enhances the speed. It is prevented from traveling backward by the refractory period, which temporarily inactivates the Na+ channels and returns the membrane to its resting state.
Step-by-step explanation:
Propagation of an Action Potential
After an action potential is generated at the trigger zone, it is propagated along the axon as voltage-gated ion channels open in response to the spreading depolarization. This process ensures that more voltage-gated Na+ channels open, and Na+ ions enter the cell, further depolarizing the adjacent cell membrane. In myelinated axons, this occurs in a saltatory conduction fashion, where the action potential jumps from one node of Ranvier to the next, greatly increasing the speed of propagation. The large diameter of the axon also aids in reducing resistance to ion flow, enhancing the rate of propagation.
Preventing Backwards Conduction
The action potential does not travel backward along the axon due to the refractory period. After an action potential occurs, the affected portion of the axon membrane enters a state where no new action potential can be generated for a short time. This is due to the inactivation of Na+ channels and the delayed opening of K+ channels, which correct the membrane potential back to its resting state.