Characteristics ofthe Action Potential

The action potential has several important characteristics that will be explained in terms of the underlying ionic permeability changes. These include the following:

1. Action potentials are triggered by depolarization. The stimulus that initiates an action potential in a neuron is a reduction in the membrane potential that is, depolarization. Normally, depolarization is produced by some external stimulus, such as the stretching of the muscle in the case of the sensory neuron in the patellar reflex, or by the action of another neuron, as in the transmission of excitation from the sensory neuron to the motor neuron in the patellar reflex.

2. A threshold level of depolarization must be reached in order to trigger an action potential. A small depolarization from the normal resting membrane potential will not produce an action potential. Typically, the membrane must be depolarized by about 10-20 mV in order to trigger an action potential. Thus, if a neuron has a resting membrane potential of about -70 mV, the membrane potential must be reduced to - 60 to - 50 mV to trigger an action potential.

3. Action potentials are all-or-none events. Once a stimulus is strong enough to reach threshold, the amplitude of the action potential is independent of the strength of the stimulus. The event either goes to completion (if depolarization is above threshold) or doesn't occur at all (if the depolarization is below threshold). In this manner, triggering an action potential is like firing a gun: the speed with which the bullet leaves the barrel is independent of whether the trigger was pulled softly or forcefully.

4. An action potential propagates without decrement throughout a neuron, but at a relatively slow speed. If we record simultaneously from the sensory fiber in the patellar reflex near the muscle and near the spinal cord, we would find that the action potential at the two locations has the same amplitude and form. Thus, as the signal travels from the muscle where it originated to the spinal cord, its amplitude remains unchanged. However, there would be a significant delay of about 0.1 sec between the occurrence of the action potential near the muscle and its arrival at the spinal cord. The conduction speed of an action potential in a typical mammalian nerve fiber is about 10-20 m/sec, although speeds as high as 100 m/sec have been observed.

5. At the peak of the action potential, the membrane potential reverses sign, becoming inside positive. As shown in Figure 6-1, the membrane potential during an action potential transiently overshoots zero, and the inside of the cell becomes positive with respect to the outside for a brief time. This phase is called the overshoot of the action potential. When the action potential repolar-izes toward the normal resting membrane potential, it transiently becomes more negative than normal. This phase is called the undershoot of the action potential.

6. After a neuron fires an action potential, there is a brief period, called the absolute refractory period, during which it is impossible to trigger another action potential. The absolute refractory period varies somewhat from one neuron to another, but it usually lasts about 1 msec. The refractory period limits the maximum firing rate of a neuron to about 1000 action potentials per second.

The goal of the remainder ofthis chapter is to explain all ofthese characteristics of the nerve action potential in terms of the underlying changes in the ionic permeability of the cell membrane and the resulting movements of ions.

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