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Unformatted text preview: 55 Chapter 6 The Conduction of the Action Potential In chapter 1, it was pointed out the reason action potentials evolved was to convey a bit of information over long distances, with high fidelity and with rapidity. The following chapters then described how action potentials are generated by the sequential opening and closing of voltage gated Na+ and K+ channels, and how those features endow axons with the ability to reproduce the action potential. As shown in Fig. 1, the reproduction of the action potential enables axons to transmit action potentials down the entire axon, regardless of its length, and with high fidelity, since the action potential that appears in any patch of membrane at one point in time is exactly the same as the action potential that appears in another patch of membrane along the axon a moment later. Fig. 1. Conduction of an action potential. Top panel : at t=1, an action potential is generated by current injected by an electrode (stimulate). A large influx of Na+ occurs at the membrane potential is driven to E Na . The positive current (indicated by red) is attracted to the negatively charged regions of the axon downstream from the action potential. Middle panel : at t=2, a moment later, the charge that moved down the axon depolarizes the axon at point B to threshold and an action potential is generated. The influx of positive current then travels to point C and at t=3 generates an action potential. The cycle is then repeated along the entire axon. The features that were not discussed are those that regulate the speed of action potential propagation. In general, there are two features of axons that determine velocity: 1) axon diameter; and 2) myelination. 56 Below I turn first to the role of axon diameter and then will explain what myelination is and why it greatly enhances conduction velocity. The larger the axon diameter, the faster the conduction velocity The increased conduction velocity with diameter was mentioned in the previous chapter when the squid giant axon was introduced. The reason why such a large axon evolved was for escape from predation. In essence, the axons rapid transmission of the signal from the animals eye to the muscles in its mantel allows for the fastest possible escape from a predator. The question that we take up next is exactly why does a larger axon create a faster conduction velocity? The explanation is that the speed with which the action potential propagates down the axon is dependent on the length of the axonal segment brought to threshold by the spread of current down the axon. The longer the advance of the current, the faster the action potential propagates. How far the local current spreads down the axon depends upon two features that act as resistances to current flow. They are; 1) the membrane resistance, and 2) the internal resistance. In other words, conduction velocity is shaped by the way membrane resistance and internal resistance change with axonal diameter....
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- Spring '08