Na Inactivation

There is one other important aspect of the data illustrated in Fig. 11. Even though the membrane potential is depolarized throughout the duration of each trace, Na+ permeability spontaneously falls back to its resting level. Thus, although Na+ permeability depends on the level of depolarization, it does not remain elevated and is only transient. Once it reaches its maximum value, it spontaneously decays back to its resting level. The process by which Na+ permeability spontaneously decays back to its resting level (even though the membrane is depolarized) is known as inactivation. At the molecular level, the process of inactivation can be considered to be a separate voltage-and time-dependent process regulating the Na+ channel. In Fig. 15, the Na+ channel is represented as having two regulatory components: an activation gate and an inactivation gate. For the channel to be open, both the activation and inactivation gates must be open. At the resting potential, the activation gate is closed, and even though the inactivation gate is open, channel permeability is zero (Fig. 15A). With depolarization, the activation gate opens rapidly (Fig. 15B), and the channel becomes permeable to Na+. Depolarization also tends to close the inactivation gate, but the inactivation process is slower. With depolarization occurring over a longer time, the inactivation gate closes, and even though the activation gate is still open, channel permeability is zero (Fig. 15C).

What is the physiologic significance of Na+ inactiva-tion? Let us return to the positive feedback cycle once again. Depolarization increases Na+ permeability, and the increase in Na+ permeability depolarizes the cell. Eventually, as a result of this regenerative cycle, the cell is depolarized rapidly up to a value near ENa. The problem with this mechanism is how to account for the repolarization phase of the action potential. Based on the relationship between Na+ permeability and membrane potential, one would predict that once the membrane potential moves to ENa it would stay there for an indefinite period of time. The steep relationship between voltage and Na+ permeability is only transient, however. After approximately 1 msec, Na+ permeability

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