Action Potentials Are Propagated Unidirectionally Without Diminution

The generation of an action potential just described relates to the changes that occur in a small patch of the neuronal

Time: 0

plasma membrane. At the peak of the action potential, passive spread of the membrane depolarization is sufficient to depolarize a neighboring segment of membrane. This causes a few voltage-gated Na+ channels in this region to open, thereby increasing the extent of depolarization in this region and causing an explosive opening of more Na+ channels and generation of an action potential. This depolarization soon triggers opening of voltage-gated K+ channels and restoration of the resting potential. The action potential thus spreads as a traveling wave away from its initial site without diminution.

As noted earlier, during the refractory period voltage-gated Na+ channels are inactivated for several milliseconds. Such previously opened channels cannot open during this period even if the membrane is depolarized due to passive spread. As illustrated in Figure 7-35, the inability of Na+ channels to reopen during the refractory period ensures that

Time: 0

Distance along axon (mm)

Distance along axon (mm)

Time: 1

Time: 1

Distance along axon (mm)

Distance along axon (mm)

Time: 2 ms

Closed

Closed

ten -50

ot p

Closed

Resting potential

Closed and Open inactivated

Closed

Closed and Open inactivated

Resting potential

Distance along axon (mm)

M FIGURE 7-35 Unidirectional conduction of an action potential due to transient inactivation of voltage-gated Na+ channels.

At time 0, an action potential (red) Is at the 2-mm position on the axon; the Na+ channels at this position are open and Na+ Ions are flowing Inward. The excess Na+ Ions diffuse In both directions along the Inside of the membrane, passively spreading the depolarization. Because the Na+ channels at the 1mm position are still Inactivated (green), they cannot yet be reopened by the small depolarization caused by passive spread; the Na+ channels at the 3-mm position, In contrast, begin to open. Each region of the membrane Is refractory (Inactive) for a few milliseconds after an action potential has passed. Thus, the depolarization at the 2-mm site at time 0 triggers action potentials downstream only; at 1 ms an action potential Is passing the 3-mm position, and at 2 ms, an action potential Is passing the 4-mm position.

al 0

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