Action potentials are initiated in the axon intial segment and then propagate forward through the axon and backwards through the dendrite.

Figure 2.  Action potentials are initiated in the initial segment of the axon in cortical pyramidal cells.  These action potentials are generated in response to the arrival of a barrage of synaptic potentials that is sufficiently depolarizing in amplitude to bring the cell above firing threshold.  These action potentials then propagate down the axon and into the dendritic arbor of the cell.  Information is thought to be communicated from cell to cell through the pattern of action potentials generated.