Conduction in Myelinated Axons

Clearly, there must be another means available by which axons can increase their propagation velocity without drastic changes in fiber diameter. You will note from the relationship for propagation velocity that by changing the membrane capacitance velocity can be affected directly without involving the square root relationship. It is possible to decrease the membrane capacitance simply by coating the axonal membrane with a thick insulating sheath. This is exactly the strategy used by the vertebrates. Many vertebrate axons are coated with a thick lipid layer known as myelin. As a result of myelin, the capacitance is greatly reduced and propagation velocity is greatly increased. In principle, there is one severe problem with increasing the propagation velocity with this technique. Coating the axon with a lipid layer would tend to cover the channels or pores in the membrane that endow the axon with the ability to initiate and propagate action potentials. The nervous system has solved this problem by coating only portions of the axon with myelin. Certain regions called nodes are not covered. At these bare regions, voltage-dependent changes in membrane permeability take place that generate action potentials.

The process of conduction in myelinated fibers is illustrated in Fig. 4. The dashed lines show a nerve axon that is covered with a layer of myelin. Note that the myelin does not cover the entire axon; there are bare regions or nodes where voltage-dependent changes in permeability can take place and action potentials can be elicited. Assume an action potential is elicited at the

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Axon Node Myelin

FIGURE 4 Saltatory conduction in myelinated axon. The electrical activity jumps from node to node. Between the nodes the action potential is propagated electronically with little decrement. At the nodes, where ionic current can flow, the electrotonic potential emerges from the internodal region, reaches threshold in the nodal region, and triggers an action potential. The process is then repeated. Arrows indicate regions of the axon where the potentials are recorded.

Axon Node Myelin

FIGURE 4 Saltatory conduction in myelinated axon. The electrical activity jumps from node to node. Between the nodes the action potential is propagated electronically with little decrement. At the nodes, where ionic current can flow, the electrotonic potential emerges from the internodal region, reaches threshold in the nodal region, and triggers an action potential. The process is then repeated. Arrows indicate regions of the axon where the potentials are recorded.

node to the left. As a result of the action potential, there is a large depolarization. The inside of the cell becomes positive with respect to the outside. The action potential cannot propagate along the myelinated region via the active process that was described earlier, simply because the voltage-dependent changes in permeability cannot take place; however, the action potential can conduct passively. That conduction will occur rapidly because the membrane capacitance is reduced. Because of the small amount of decrement, the potential that "emerges" at the next node will be of a sufficient level to depolarize the next node to threshold. A new action potential will be initiated, and the process will be repeated.

The type of propagation that occurs in myelinated fibers is known as saltatory conduction because the action potential appears to jump from node to node. At the nodes, there are voltage-dependent changes in membrane permeability, whereas in the internodal regions the potential is conducted in a passive fashion. No voltage-dependent changes in permeability take place in the internodal region.

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