If summation of all the excitatory and inhibitory graded potentials at any moment in time brings the membrane potential of the trigger zone over the threshold value of around -50 mV, an action potential will usually be fired down the axon. This is the way that neurons transmit information over a wide range of distances, which may be one meter or more. Action potentials differ from graded potentials in that they usually have the same size and duration for any particular neuron, and they are usually conducted along the axon unchanged regardless of the distance.
Action potentials start with a rapid membrane potential change from the threshold potential of around -50 mV to a peak of around +40 mV, followed immediately by a rapid change to around -70 mV, and then a slower return to the resting potential of around -60 mV. Although the size of the potential change may vary between neurons, is usually consistent for each neuron, which is called its all-or-none property, as opposed to graded potentials, where the size varies depending on the size of the input. The action potential will be the same size regardless of if the depolarizations that caused it are just slightly over the threshold potential or if they are way over it.
The short duration of action potentials is usually just a few milliseconds, which is also different from graded potentials which may have a much longer duration depending on the duration of the input. The action potential waveform is usually conducted unchanged from the trigger zone to the axon terminals at speeds ranging from one to 100 meters per second. Large diameter axons and myelinated axons conduct action potentials faster than small diameter and unmyelinated axons. For myelinated axons, the action potential moves faster through each myelinated segment, and slower through each node of Ranvier. This is called saltatory conduction from a Latin word for jumping, because it appears to jump from node to node.
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Introduction to Neurology 