Initiation and Propagation of Action Potentials

Some of the fundamental properties of action potentials can be studied experimentally using an apparatus like that diagrammed in Figure 6-2a. Imagine that a long section of a single axon is removed and arranged in the apparatus so that intracellular probes can be placed inside the fiber at two points, a and b, which are 10 cm apart. The probe at a is set up to pass positive or negative charge into the fiber and to record the resulting change in membrane potential, while the probe at b records membrane potential only. The effect of injecting negative charge at a constant rate at a is shown in Figure 6-2b. The extra negative charges make the interior of the fiber more negative, and the membrane potential increases; that is, the membrane is hyperpolarized. At the same time, the probe at b records no change in membrane potential at all, because the plasma membrane is leaky to charge. In Chapter 3, we discussed the cell membrane as an electrical capacitor. In addition, the membrane behaves like an electrical resistor; that is, there is a direct path through which ionic current may flow across the membrane. As we saw in Chapter 5, that current path is through the ion channels that are inserted into the lipid bilayer of the plasma membrane. Thus, the charges injected at a do not travel very far down the fiber

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Figure 6-2 The generation and propagation of an action potential in a nerve fiber. (a) Apparatus for recording electrical activity of a segment of a sensory nerve fiber. The probes at points a and b allow recording of membrane potential, and the probe at a also allows injection of electrical current into the fiber. (b) Injecting negative charges at a causes hyperpolarization at a. All injected charges leak out across the membrane before reaching b, and no change in membrane potential is recorded at b.

(c) Injection of a small amount of positive charge produces a depolarization at a that does not reach b.

(d) If a stronger depolarization is induced at a, an action potential is generated. The action potential propagates without decrement along the fiber and is recorded at full amplitude at b.

Figure 6-3 A schematic representation of the decay of injected current in an axon with distance from the site of current injection.

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before leaking out of the cell across the plasma membrane. None of the charges reaches b, and so there is no change in membrane potential at b. When we stop injecting negative charges at a, all the injected charge leaks out of the cell, and the membrane potential returns to its normal resting value. The electrical properties of cells and the response to charge injection are described in more detail in Appendix C.

Another way of looking at the situation in Figure 6-2b is in terms of the flow of electrical current. The negative charges injected into the cell at a constant rate constitute an electrical current originating from the experimental apparatus. The return path for the current to the apparatus lies in the ECF, so that in order to complete the circuit the current must exit across the plasma membrane. Two paths are available for the current at the point where it is injected: it can flow across the membrane immediately or it can move down the axon to flow out through a more distant segment of axon membrane. This situation is illustrated in Figure 6-3 (also see Appendix C). The injected current will thus divide, some taking one path and some the other. The proportion of current taking each path depends on the relative resistances of the two paths: more current will flow down the path with less resistance. With each increment in distance along the axon, that fraction of the injected current that flowed down the axon again faces two paths; it can continue down the interior of the axon or it can cross the membrane at that point. The current will again divide, and some fraction of the remaining injected current will continue down the nerve fiber. This process will continue until all the injected current has crossed the membrane, and no current is left to flow further down the interior of the axon. At that point, the injected current will not influence the membrane potential because there will be no remaining injected current. Thus, the change in membrane potential produced by current injection (Figure 6-2a) decays with distance from the injection site. The greatest effect occurs at the injection site, and there is progressively less effect as injected current is progressively lost across the plasma membrane. Appendix C presents a quantitative discussion of this decay of voltage with distance along a nerve fiber. The cell membrane is not a particularly good insulator (it has a low resistance to current flow compared, for example, with the insulator surrounding the electrical wires in your house), and the ICF inside the axon is not a particularly good conductor (its resistance to current flow is high compared with that of a copper wire). This set of circumstances favors the rapid decay of injected current with distance. In real axons, the hyperpolarization produced by current injected at a point decays by about 95% within 1-2 mm of the injection site.

Let's return now to the experiment shown in Figure 6-2. The effect of injecting positive charges into the axon is shown in Figure 6-2c. If the number of positive charges injected is small, the effect is simply the reverse of the effect of injecting negative charges; the membrane depolarizes while the charges are injected, but the effect does not reach b. When charge injection ceases, the extra positive charges leak out of the fiber, and membrane potential returns to normal. If the rate of injection of positive charge is increased, as in Figure 6-2d, the depolarization is larger. If the depolarization is sufficiently large, an all-or-none action potential, like that recorded when the muscle was stretched (Figure 6-1), is triggered at a. Now, the probe at b records a replica of the action potential at a, except that there is a time delay between the occurrence of the action potential at a and its arrival at b. Thus, action potentials are triggered by depolarization, not by hyperpolarization (characteristic 1, above), the depolarization must be large enough to exceed a threshold value (characteristic 2), and the action potential travels without decrement throughout the nerve fiber (characteristic 4). What ionic properties of the neuron membrane can explain these properties?

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