Propagation of an Action Potential Along a Nerve Fiber

We can now see how an action potential arises as a result of a depolarizing stimulus, such as the muscle stretch in the case of the sensory neuron in the patellar reflex. How does that action potential travel from the ending in the muscle along the long, thin sensory fiber to the spinal cord? The answer to this question is inherent in the scheme for generation ofthe action potentialjust presented. As we've seen, the stimulus for an action potential is a depolarization of greater than about 10-20 mV from the normal resting level of membrane potential. The action potential itself is a depolarization much in excess of this threshold level. Thus, once an action potential occurs at one end of a neuron, the strong depolarization will bring the neighboring region of the cell above threshold, setting up a regenerative depolarization in that region. This will in turn bring the next region above threshold, and so on. The action potential can be thought of as a self-propagating wave of depolarization sweeping along the nerve fiber. When the sequence of permeability changes summarized in Figure 6-8 occurs in one region of a nerve membrane, it guarantees that the same gating events will be repeated in neighboring segments of membrane. In this manner, the cyclical changes in membrane permeability, and the resulting action potential, chews its way along the nerve fiber from one end to the other, as each segment of axon membrane responds in turn to the depolarization of the preceding segment. This behavior is analogous to that of a lighted fuse, in which the heat generated in one segment of the fuse serves to ignite the neighboring segment.

A more formal description of propagation can be achieved by considering the electrical currents that flow along a nerve fiber during an action potential. Imagine that we freeze an action potential in time while it is traveling down an axon, as shown in Figure 6-9a. We have seen that at the peak of the action potential, there is an inward flow of current, carried by sodium ions. This is shown by the inward arrows at the point labeled 1 in Figure 6-9a. The region of axon occupied by the action potential will be depolarized with respect to more distant parts of the axon, like those labeled 2 and 3. This difference in electrical potential means that there will be a flow of depolarizing current leaving the depolarized region and flowing along the inside of the nerve fiber; that is, positive charges will move out from the region of depolarization. In the discussion of the response to injected current in an axon (Figures 6-2 and 6-3), we saw that a voltage change produced by injected current decayed with distance from the point of injection. Similarly, the depolarization produced by the influx of sodium ions during an action potential will decay with distance from the region of membrane undergoing the action potential. This decay of depolarization with distance reflects the progressive leakage of the depolarizing current across the membrane, which occurs because the membrane is a leaky insulator. Figure 6-9b illustrates the profile of membrane potential that might be

Figure 6-9 The decay of depolarization with distance from the peak of the action potential at a particular instant during the propagation of the action potential from left to right along the axon.

Figure 6-9 The decay of depolarization with distance from the peak of the action potential at a particular instant during the propagation of the action potential from left to right along the axon.

observed along the length of the axon at the instant the action potential at point 1 reaches its peak. Note that there is a region of axon over which the depolarization, although decaying, is still above the threshold for generating an action potential in that part of the membrane. Thus, if we "unfreeze" time and allow events to move along, the region that is above threshold will generate its own action potential. This process will continue as the action potential sweeps along the axon, bringing each successive segment of axon above threshold as it goes.

The flow of depolarizing current from the region undergoing an action potential is symmetrical in both directions along the axon, as shown in Figure 6-9a. Thus, current flows from point 1 to both point 2 and to point 3 in the figure. Nevertheless, the action potential in an axon typically moves in only one direction. That is because the region the action potential has just traversed, like point 3, is in the refractory period phase of the action potential cycle and is thus incapable of responding to the depolarization originating from the action potential at point 1. Of course, if a neurophysiologist comes along with an artificial situation, like that shown in Figure 6-2, and stimulates an action potential in the middle of a nerve fiber, that action potential will propagate in both directions along the fiber. The normal direction of propagation in an axon the direction taken by normally occurring action potentials is called the orthodromic direction; an abnormal action potential propagating in the opposite direction is called an antidromic action potential.

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