Refers to the phenomenon where the strength of a nerve impulse is not dependent on the strength of the stimulus. Instead, there is a threshold level of stimulus strength that must be reached before the nerve will fire an impulse (at full capacity). Below the threshold, the nerve will not fire at all.In cardiology, it refers to the same phenomenon observed in the heart muscle, which will either contract fully or not at all.In studies of behavior, it refers to the same phenomenon where a behavioral stimulus will either produce a complete response or no response at all. Also called all-or-nothing principle, all-or-none law, all-or-none responsiveness, etc.