AIIor None

Once a region of axon membrane has been depolarized to a threshold value, the positive feedback effect of depolarization on

Na+ permeability and of Na+ permeability on depolarization causes the membrane potential to shoot toward about +30 mV. It does not normally become more positive than +30 mV because the Na+ channels quickly close and the K+ channels open. The length of time that the Na+ and K+ channels stay open is independent of the strength of the depolarization stimulus.

The amplitude (size) of action potentials is therefore all or none. When depolarization is below a threshold value, the voltage-regulated gates are closed; when depolarization reaches threshold, a maximum potential change (the action potential) is produced. Since the change from -70 mV to +30 mV and back to -70 mV lasts only about 3 msec, the image of an action potential on an oscilloscope screen looks like a spike. Action potentials are therefore sometimes called spike potentials.

The channels are only open for a fixed period of time because they are soon inactivated, a process different from simply closing the gates. Inactivation occurs automatically and lasts until the membrane potential has repolarized. Because of this automatic inactivation, all action potentials have about the same duration. Likewise, since the concentration gradient for Na+ is

Action potential recording /

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Action potential recording /

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Time

■ Figure 7.15 The effect of stimulus strength on action potential frequency. These are recordings from a single sensory fiber of the sciatic nerve of a frog stimulated by varying degrees of stretch of the gastrocnemius muscle. Notice that increasing degrees of stretch (indicated by increasing weights attached to the muscle) result in a higher frequency of action potentials.

relatively constant, the amplitudes of the action potentials are about equal in all axons at all times (from -70 mV to +30 mV, or about 100 mV in total amplitude).

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