The refractory period of a neuron is a phase during which the neuron cannot generate another action potential immediately after one has occurred. This period ensures that the action potential cannot travel backward because the region of the membrane that just fired is temporarily inexcitable. Consequently, the impulse propagates unidirectionally along the axon, moving from the axon hillock toward the synaptic terminals. This mechanism is crucial for the orderly transmission of neural signals and prevents the signal from disrupting previously activated areas.

Reference: Principles of Neural Science, Kandel et al., 5th Edition.