Final answer:
An action potential is initiated when the cell membrane reaches the threshold potential, leading to a rapid depolarization and subsequent steps resulting in a full cycle of action potential that restores the cell to its resting state.
Step-by-step explanation:
An action potential will be initiated if the membrane reaches threshold. The process of action potential occurs in several steps. Initially, a stimulus causes depolarization of the membrane, moving toward the threshold potential. If this threshold of excitation is achieved, with a typical depolarization reaching around -55 mV, all Na+ channels open, causing the membrane to rapidly depolarize. The peak of this action potential occurs around +30 mV, at which point K+ channels open, and the Na+ channels have already closed. This results in repolarization as K+ leaves the cell. Following this, the cell becomes hyperpolarized, entering a refractory period in which it can't fire another action potential. Lastly, K+ channels close, and through the activity of the Na+/K+ pump, the resting potential is restored. The action potential operates on an all-or-nothing principle. The response is not proportional to the strength of the stimulus; a stronger stimulus will result in more frequent action potentials, but not larger ones.