The speed with which an action potential moves down an axon varies considerably from one axon to another; the range is from about 0.1 m/sec to 100 m/sec. What characteristics of an axon are important in the determining the action potential propagation velocity? Examine Figure 6-9b again. Clearly, if the rate at which the depolarization falls off with distance is less, the region of axon brought above threshold by an action potential at point 1 will be larger. If the region above threshold is larger, then an action potential at a particular location will set up a new action potential at a greater distance down the axon and the rate at which the action potential moves down the fiber will be greater. The rate of voltage decrease with distance will in turn depend on the relative resistance to current flow of the plasma membrane and the intracellular path down the axon. Recall from the discussion of the response of an axon to injection of current (see Figure 6-3) that there are always two paths that current flowing down the inside of axon at a particular point can take: it can continue down the interior of the fiber or cross the membrane at that point. We said that the portion of the current taking each path depends on the relative resistances of the two paths. If the resistance of the membrane could be made higher or if the resistance ofthe path down the inside of the axon could be made lower, the path down the axon would be favored and a larger portion of the current would continue along the inside. In this situation, the depolarization resulting from an action potential would decay less rapidly along the axon; therefore, the rate of propagation would increase.
Thus, two strategies can be employed to increase the speed of action potential propagation: increase the electrical resistance of the plasma membrane to current flow, or decrease the resistance of the longitudinal path down the inside of the fiber. Both strategies have been adopted in nature. Among invertebrate animals, the strategy has been to decrease the longitudinal resistance of the axon interior. This can be accomplished by increasing the diameter of the axon. When a fiber is fatter, it offers a larger cross-sectional area to the internal flow of current; the effective resistance of this larger area is less because the current has many parallel paths to choose from if it is to continue down the interior of the axon. For the same reason, the electric power company uses large-diameter copper wire for the cables leaving a power-generating station; these cables must carry massive currents and thus must have low resistance to current flow to avoid burning up. Some invertebrate axons are the neuronal equivalent of these power cables: axons up to 1 mm in diameter are found in some invertebrates. As expected, these giant axons are the fastest-conducting nerve fibers of the invertebrate world.
Among vertebrate animals, there is also large variation in the size of axons, which range from less than 1 ^m in diameter to as big as 30-50 ^m in diameter. Thus, even the largest axons in a human nerve do not begin to rival the size of the giant axons of invertebrates. Nevertheless, the fastest-conducting vertebrate axons are actually faster than the giant invertebrate axons. Vertebrate animals have adopted the strategy of increasing the membrane resistance to current as well as increasing internal diameter. This has been accomplished by wrapping the axon with extra layers of insulating cell membrane: in order to reach the exterior, electrical current must flow not only through the resistance of the axon membrane, but also through the cascaded resistance of the tightly wrapped layers of extra membrane. Figure 6-10a shows a schematic cross-section of a vertebrate axon wrapped in this way. The cell that provides the spiral of insulating membrane surrounding the axon is a type of glial cell, a
Figure 6-10 The propagation of an action potential along a myelinated nerve fiber. (a) Cross-section of a myelinated axon, showing the spiral wrapping of the glial cell membrane around the axon. (b) The depolarization from an action potential at one node spreads far along the interior of the fiber because the insulating myelin prevents the leakage of current across the plasma membrane. (Animation available at www.blackwellscience.com)
non-neuronal supporting cell of the nervous system that provides a sustaining mesh in which the neurons are embedded.
The insulating sheath around the axon is called myelin. By increasing the resistance of the path across the membrane, the myelin sheath forces a larger portion of the current flowing as the result of voltage change to move down the interior of the fiber. This increases the spatial spread of a depolarization along the axon and increases the rate at which an action potential propagates. In order to set up a new action potential at a distant point along the axon, however, the influx ofsodium ions carrying the depolarizing current during the initiation of the action potential must have access to the axon membrane. To provide that access, there are periodic breaks in the myelin sheath, called nodes of Ranvier, at regular intervals along the length of the axon. This is diagrammed in Figure 6-10b. Thus, the depolarization resulting from an action potential at one node of Ranvier spreads along the interior of the fiber to the next node, where it sets up a new action potential. The action potential leaps along the axon, jumping from one node to the next. This form of action potential conduction is called saltatory conduction, and it produces a dramatic improvement in the speed with which a thin axon can conduct an action potential along its length.
The myelin sheath also has an effect on the behavior of the axon as an electrical capacitor. Recall from Chapter 3 that the cell membrane can be viewed as an insulating barrier separating two conducting compartments (the ICF and ECF). Thus, the cell membrane forms a capacitor. The capacitance, or charge-storing ability, of a capacitor is inversely related to the distance between the conducting plates: the smaller the distance, the greater the number of charges that can be stored on the capacitor in the presence of a particular voltage gradient. Thus, when the myelin sheath wrapped around an axon increases the distance between the conducting ECF and ICF, the effective capacitance of the membrane decreases. This means that a smaller number of charges needs to be added to the inside of the membrane in order to reach a particular level of depolarization. (If it is unclear why this is true, review the calculation in Chapter 3 of the number of charges on a membrane at a particular voltage.) An electrical current is defined as the rate of charge movement that is, number of charges per second. In the presence of a particular depolarizing current, then, a given level of voltage will be reached faster on a small capacitor than on a large capacitor. Because the myelin makes the membrane capacitance smaller, a depolarization will spread faster, as well as farther, in the presence of myelin.
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