Threshold
Imagine trying to start a campfire with damp wood. Just as you need a certain amount of heat from a match or lighter to finally get the flames going, a nerve requires a minimum level of stimulus to trigger an impulse. The campfire represents the nerve, the heat needed to ignite it is like the stimulus, and only when you reach that threshold does the fire—or the impulse—begin.
Practice Version
Threshold: The minimum stimulus needed to produce a nerve impulse is called the threshold. In science, threshold is the point at which a stimulus is strong enough to trigger a response.