What triggers at a point in which neuron will fire?

Action potential

When the membrane potential reaches a certain threshold, an action potential is triggered. This causes the membrane to depolarize, which means that the inside of the cell becomes more positive relative to the outside. This change in membrane potential is caused by the opening of sodium channels, which allows sodium ions to flow into the cell. The influx of sodium ions causes the membrane potential to become more positive, and if the threshold is reached, an action potential is triggered.