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Action potentials are conducted along axons without loss of signal

An AP is conducted over long distances with no loss of signal. If we place two pairs of electrodes at two different locations along an axon, we can record an AP at those two locations as it travels along the axon (Figure 44.9A). The magnitude of the AP does not change between the two recording sites. This constancy is possible because an AP is an all-or-none, self-regenerating event.

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An AP is all-or-none because of the interaction between the voltage-gated Na⁺ channels and the membrane potential. If the membrane is depolarized slightly, some voltage-gated Na⁺ channels open. Some sodium ions cross the cell membrane and depolarize it even more, opening more voltage-gated Na⁺ channels, and so on, generating an AP. This positive feedback mechanism ensures that APs always rise to their maximum value.
An AP is self-regenerating because it spreads by local current flow to adjacent regions of the cell membrane. The resulting depolarization brings those neighboring areas of membrane to threshold. Therefore, when an AP occurs at one location on an axon, it stimulates the adjacent region of axon to generate an AP, and so on down the length of the axon. Although we say that the AP travels down the axon, what is really happening is that new APs are being created sequentially.

Figure 44.9 Action Potentials Travel along Axons (A) There is no loss of signal as an action potential travels along an axon. (B) When an action potential is stimulated in one region of membrane, ionic current flows to and depolarizes adjacent areas of membrane. (C) The advancing wave of depolarization causes more Na⁺ channels to open, and the action potential is generated anew in the next section of membrane. Meanwhile, in the region where the action potential has just fired, the Na⁺ channels are inactivated and the voltage-gated K⁺ channels are still open, rendering this section of the axon incapable of generating an action potential. Hence the action potential cannot “back up,” but moves continuously forward, regenerating itself as it goes.

We can use an electrode to stimulate an axon, causing it to depolarize and to fire an AP that is then conducted along the axon. Figure 44.9B shows the changes in the ion channels in the membrane that are responsible for conducting the AP along the axon without a reduction in amplitude. Normally an AP is propagated in only one direction—away from the cell body. It cannot reverse itself because the voltage-gated Na⁺ channels in the region of the membrane it came from are in their refractory period (Figure 44.9C).

APs are not conducted at the same speed in all axons. They travel faster in large-diameter axons than in small-diameter axons because the resistance to ionic current flow decreases as an axon’s diameter gets bigger. They travel faster in myelinated than in nonmyelinated axons because they can move down the axon in short “jumps” as described in the next section (Figure 44.10). Invertebrates depend on increased axon diameter for fast conduction, but vertebrates mostly depend on myelination of axons to increase conduction velocity.

Figure 44.10 Saltatory Action Potentials Action potentials “jump” from node to node in myelinated axons, allowing faster transmission of information.