Closer investigation has revealed that nerve impulses involve both electrical and chemical changes. The rate at which these impulses travel along a nerve is considerably less than the rate at which an electric current travels. When the activity of a single neuron is measured, we fine that neuron possesses a characteristic that psychologists refer to as the “threshold of excitability.” If we stimulate the cell with increasingly stronger stimuli, we find a point at which the neuron transmits or “fires” an impulse.

With stimuli below this threshold, the neuron remains inert, but as soon as we increase the intensity of the stimulation above a certain point, the neuron transmits or discharges. The significant point is that the neuron reacts in the same way to stimuli above the threshold, regardless of the intensity of the stimulus applied. The phenomenon is known as the “all-or-none law” and it should be noted that although the law applies to the axon, dendrites are characterized by graded potentials, with strength of the potential varying from time to time.

A stimulus, then must be of certain intensity and above the neuron’s threshold before the neuron will fire. Secondly, neurons react in the same way to stimuli of varying intensity, as long as they are above threshold. A neuron, in other words, either fires or fails to fire, depending on whether the stimulus applied to it above or below the threshold.