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The biologists working with the squid giant axon notices something strange. When they stimulated the axon with an electric shock, it set off a much larger electrical impulse, a spike in voltage that traveled down the axon in a wave without losing any intensity along the way. This electric impulse is called an action potential, an electric signal that is conducted along the length of a neuron's axon to a synapse. The action potential isn't just some curious feature of the squid giant axon, it is a fundamental basis for just about everything that we think, feel, or do, and so it is important to understand how it works.
The biologists Alan Hodgkin and Andrew Huxley discovered the resting potential in the summer of 1993 while studying marine invertebrates, sea creatures that lack a spine, such as clams, squid, and lobsters. Hodgkin and Huxley worked with the squid giant axon because it is 100 times larger than the biggest axon in humans. They inserted a thin wire into the squid axon so that it touched the jellylike fluid inside. Then they placed another wire just outside the axon in the watery fluid that surrounds is. They found a substantial difference between the electric charges inside and outside the axon, which they called the resting potential.
The action potential occurred only when the electric shock reached a certain level, or threshold. Above that threshold, increases in the electric shock did not increase the strength of the action potential. The action potential is all or none: Electric stimulation below the threshold fails to produce an action potential, whereas electric stimulation at or above the threshold always produces the action potential and always at the same strength. Because of the rapid, all-or-nothing nature of an action potential, scientists sometimes say that a neuron "fires" when it produces an action potential. (By way of analogy, a revolver ejects one bullet at a constant velocity whenever the trigger is pulled with sufficient force. Pulling on the trigger with sufficient force causes the revolver to fire but, beyond that point extra force on the trigger will not cause the bullet to be ejected more forcefully or at greater speed; the firing event is all-or-nothing.)