Question

why nerve impulse flow only in one direction

Answer 1

This is electrochemical energy.

Neurotransmitters are molecules that fit like a lock and key into a specific receptor. Therefore, nerve impulses cannot travel in the opposite direction, because nerve cells only have neurotransmitter storage vesicles going one way, and receptors in one place.

Answer 2

This is because of the presence of synapse between two nerve cells......
they are like valves which allow only unidirectional flow of the chemicals passed by the preceding nerve ending
Hope this helps ☺