Why is the flow of signals in a synapse from axonal end of one neuron to dendritic end of another neuron but not the reverse?

Explanation

The synapse actually act like a one-way valve because the chemical substance is present only on one side of the gap. This chemical diffuses towards the dendrite end of next neuron where it generates an electrical signal.
Since, the chemicals are absent at the dendritic end of neuron, the nerve impulse can go across only from one side (which contains the chemical substance). In this way, it is ensured that nerve impulses travel in only one direction (through of particular set of neurons).