If a stimulus is strong enough to destabilize the neuronal membrane, the neuron produces an action potential; in that case, we say the neuron has fired. The action potential will be the same size and is propagated at the same speed, regardless of whether the stimulus just meets threshold or exceeds it by 2, 3, or 20 times. This phenomenon is sometimes called the all-or-none law. Just as pounding on a car horn won’t make it any louder, a stronger stimulus won’t produce a stronger action potential. A neuron either fires or it doesn’t—there’s no in between.
Obviously, though, we need some way to differentiate the weak signals from the strong. Otherwise, we’d have no way to tell apart the buzz of a mosquito and the roar of a jet engine, the light of a distant candle and the full illumination of a sunny day. If neurons can’t vary the strength of their response, how do we make these differentiations?
Part of the answer is that more intense stimuli excite greater numbers of neurons. This happens because neurons vary enormously in their excitation thresholds. As a result, a weak stimulus stimulates only neurons with relatively low thresholds, while a strong stimulus stimulates all of those neurons plus others whose threshold is higher.
It’s also important to realize that when neurons are bombarded with a sustained stimulus, they do not just fire once and then stop. Instead they generate a whole stream, or “volley,” of action potentials by means of repeated cycles of destabilization and resta-bilization. Of course, the all-or-none law applies within the volley, so the size of each action potential is always the same. Even so, neurons can vary the rate of their firing in the volley; and in most cases, the stronger the stimulus, the more often the neuron will fire. This pattern holds until we reach a maximum rate of firing, after which further increases in stimulus intensity have no effect (Figure 3.12). Different neurons have dif-ferent maximum rates, and the highest in humans are on the order of 1,000 impulses per second.