The rule that neurons are either on or off is known as the "all-or-none law." According to the all-or-none law, a neuron will either fire an action potential at full strength or not fire at all. In other words, there is no partial firing of an action potential. When the neuron's membrane potential reaches a certain threshold, an action potential is generated and propagates along the neuron. If the threshold is not reached, no action potential is generated. This principle applies to individual neurons and their firing patterns. However, in a network of interconnected neurons, the combined activity of multiple neurons can give rise to graded responses and varying levels of neural activity.