all-or-none law, 1. the principle in neurophysiology stating that a stimulus must be strong enough to reach threshold to trigger a nerve impulse. Once theshold is achieved, the entire impulse is discharged. A weak stimulus will not produce a weak reaction. 2. the principle that the heart muscle or nerve, under any stimulus above a threshold level, will respond either with a maximal strength response or with none at all. Also called Bowditch’s law.
