What is the chemical that causes action potential

The inward flow of sodium ions which concentrates a positive reaction in the cell, ultimately leading to the depolarization of the cell. This causes raising the potential higher the usual rate of the cell. This causes the sodium channels to close at their peak while potassium continuously flows out of the cell.
For a more in-depth explanation, I would recommend this web page : https://www.macalester.edu/academics/psychology/whathap/ubnrp/meth08/biochemistry/neurotransmission