Answer:
From the aspect of ions, an action potential is caused by temporary changes in membrane permeability for diffusible ions. These changes cause ion channels to open and the ions to decrease their concentration gradients.
The ion substitution experiments carried out by Hodgkin and Katz provided convincing evidence that the resting membrane potential results from a high resting membrane permeability to K+, and that depolarization during an action potential results from a transient rise in membrane Na+ permeability.
The principal ions involved in an action potential are sodium and potassium cations; sodium ions enter the cell, and potassium ions leave, restoring equilibrium.