An action potential occurs at one point along the axon. Yet we know that neurological impulses are not fixed, they travel along a neuron. So how can the action potential at one point excite a neuron at another point? The action potential itself is also a stimulus. The Na+ that rush in the axon after depolarisation are conducted by the cable properties (the ability of a neuron to transmit charges through the cytoplasm) to the adjacent region that still has a membrane potential of -70mV The limits of transmitting electric charge through cytoplasm is one to two millimetres.
When the stimulus is of threshold value in the adjacent region it too produces an action potential. Therefore, an action potential is a self propagating event.