If a stimulus of a given intensity is maintained at one point of an axon and depolarizes it to threshold, action potentials will be produced at that point at a given frequency (number per second). As the stimulus strength is increased, the frequency of action potentials produced at that point will increase accordingly. As action potentials are produced with increasing frequency, the time between successive action potentials will decrease—but only up to a minimum time interval. The interval between successive action potentials will never become so short as to allow a new action potential to be produced before the preceding one has finished.
During the time that a patch of axon membrane is producing an action potential, it is incapable of responding—or refractory— to further stimulation. If a second stimulus is applied during most of the time that an action potential is being produced, the second stimulus will have no effect on the axon membrane. The membrane is thus said to be in an absolute refractory period; it cannot respond to any subsequent stimulus.
The cause of the absolute refractory period is now understood at a molecular level. In addition to the voltage-regulated gates that open and close the channel, an ion channel may have a polypeptide that functions as a "ball and chain" apparatus dangling from its cytoplasmic side (see fig. 7.12). After a voltage-regulated channel is opened by depolarization for a set time, it enters an inactive state. The inactivated channel cannot be opened by depolarization. The reason for its inactivation depends on the type of voltage-gated channel. In the type of channel shown in fig. 7.12, the channel becomes blocked by a molecular ball attached to a chain. In a different type of voltage-gated channel, the channel shape becomes altered through molecular rearrangements. The inactivation ends after a fixed period of time in both cases, either because the ball leaves the mouth of the channel, or because molecular rearrangements restore the resting form of the channel. In the resting state, unlike the inactivated state, the channel is closed but it can be opened in response to a depolarization stimulus of sufficient strength.
If a second stimulus is applied while the K+ gates are open (and the membrane is in the process of repolarizing), the membrane is said to be in a relative refractory period. During this time, only a very strong depolarization can overcome the repo-larization effects of the open K+ channels and produce a second action potential (fig. 7.16).
Because the cell membrane is refractory during the time it is producing an action potential, each action potential remains a separate, all-or-none event. In this way, as a continuously applied stimulus increases in intensity, its strength can be coded strictly by the frequency of the action potentials it produces at each point of the axon membrane.
After a large number of action potentials have been produced, one might think that the relative concentrations of Na+ and K+ would be changed in the extracellular and intracellular compartments. This is not the case. In a typical mammalian axon that is 1 mm in diameter, for example, only one intracellu-lar K+ in 3,000 would be exchanged for a Na+ to produce an action potential. Since a typical neuron has about 1 million Na+/K+ pumps that can transport nearly 200 million ions per second, these small changes can be quickly corrected.
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