Applied to the axon (single, relatively lengthy process) of a nerve cell or neuron, the all-or-none law states that transmission of a nerve impulse occurs either all the way or not at all. If the changes that produce the nerve impulse—that is, the movement of charged particles or ions—reach a certain threshold level, then the impulse (also called the action potential or spike potential) is conducted at a constant level from its origin to the end of the axon.

Another way the law is sometimes expressed is that ax-onal transmission is independent of the intensity of the stimulus that produces it. As long as the stimulus causes enough ionic movement to exceed a threshold, the nerve impulse occurs all the way, without decreasing as it travels the length of the axon. A mild stimulus that surpasses the threshold produces the same nerve impulse as an intense stimulus. The nervous system codes the intensity of a stimulus by the rate of generation of action potentials, not by whether they occur, and also by the number of neurons activated in a given area. The greater the intensity, the larger the number of neurons activated and the more rapidly they generate action potentials. A neuron's action potentials are analogous to signals from a telegraph key: Aneuron cannot send bigger or faster action potentials any more than a telegraph operator can send bigger or faster signals with the telegraph key.

The all-or-none concept applies to other excitable tissue as well, and the principle was first demonstrated in 1871 in heart muscle by American physiologist Henry P. Bowditch. In 1902, English physiologist F. Gotch discovered evidence for an all-or-none effect in nerves, but the effect was not convincingly proven until Edgar Douglas Adrian's work, for which he received a Nobel prize in physiology in 1932. Adrian's research was preceded by studies performed by K. Lucas, who actually named the law in a 1909 article.

Like most of the nervous system's so-called laws, the all-or-none law has exceptions. For example, some neurons can produce a series of action potentials that grow successively smaller, thus disobeying the law.

B. Michael Thorne

Mississippi State University

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