An action potential will not occur unless the membrane potential at the (the initial segement of the axon) reaches a level called .

An action potential will not occur unless the membrane potential AT THE VOLTAGE GATED SODIUM CHANNEL reaches a level called THRESHOLD.
An action potential refers to the change in the electrical potential which occurs as a result of the passage of impulses along the membrane of muscle cells or nerve cells. An action potential occurs when there is depolarization of the cell membrane to the threshold stage.

Dthrough a slow process