Evolution or action potential.

[gianeshwar]

Action potential is defined as"
the local voltage change across the cell wall as a nerve impulse is transmitted"
Now my question is "why it happens? i.e.,why organism evolves?

 

[Pythagorean]

A little bit of luck (mutation) provided a benefit in organism function that allowed organisms to reproduce, propagating that function (selection). Ionic currents in cells (and associated action potentials) were already around before cells started taking advantage of them at the level neurons do.

You can read a bit about the evolutionary history on the wiki:

http://en.wikipedia.org/wiki/Evolution_of_nervous_systems

 

[gianeshwar]

Thank you Pythagorean.I have studied the link you sent .Will come back after more study.
One thing you referred mutation as luck is difficult to justify.