Final answer:
An action potential is propagated along a neuron's axon by the sequential opening of voltage-gated sodium channels, allowing the inward flow of sodium ions, followed by the opening of potassium channels to repolarize the segment. This process moves like a wave in one direction down the axon.
Step-by-step explanation:
An action potential spreads along the cell membrane primarily through the opening of voltage-gated sodium channels. When a region of the membrane depolarizes, it triggers adjacent voltage-gated sodium channels to open, allowing more sodium ions to flow into the neuron. This influx of positive charge depolarizes the next segment of the membrane, which then opens more sodium channels in a chain reaction down the length of the neuron's axon.
Once the inside of the membrane becomes sufficiently positive, voltage-gated potassium channels open, enabling potassium ions to leave the cell, repolarizing and eventually hyperpolarizing the membrane, which ends the action potential in that segment. Sodium channels then reset to their resting state, and potassium channels close after a delay, restoring the resting potential. The wave of depolarization can only travel in one direction as the recently activated sodium channels enter an inactivated state and will not open again until they have returned to their resting state.