Myelinated Fibers

Matters are somewhat different in myelinated fibers, because voltage-regulated ion gates are scarce in the myelin-covered internodes—fewer than 25 per ^m2 in these regions compared with 2,000 to 12,000 per ^m2 at the nodes of Ranvier. There would be little point in having ion gates in the internodes—myelin insulates the fiber from the ECF at these points, and sodium ions from the ECF could not flow into the cell even if more gates were present.

The only way a nerve signal can travel along an internode is for Na+ that enters at the previous node to diffuse down the fiber under the axolemma (fig. 12.15a). This is a very fast process, but the nerve fiber resists their flow (just as a wire resists a current) and the signal becomes weaker the farther it goes. Therefore, this aspect of conduction is decremental. The signal cannot travel much farther than 1 mm before it becomes too weak to open any voltage-regulated gates. But fortunately, there is another node of Ranvier every millimeter or less along the axon, where the axolemma is exposed to ECF and there is an abundance of voltage-regulated gates. When the diffusing ions reach this point, the signal is just strong enough to open these gates and create a new action potential. This action potential has the same strength as the one at the previous node, so each node of Ranvier boosts the signal back to its original strength ( + 35 mV). This mode of signal conduction is called saltatory25 conduction— the propagation of a nerve signal that seems to jump from node to node (fig. 12.15b).

In the internodes, saltatory conduction is therefore based on a process that is very fast (diffusion of ions along the fiber) but decremental. In the nodes, conduction is slower but nondecremental. Since most of the axon is covered with myelin, conduction occurs mainly by the fast longitudinal diffusion process. This is why myeli-nated fibers transmit signals much faster (up to 120 m/sec) than unmyelinated ones (up to 2 m/sec) and why the signal is just as strong at the end of the fiber as it was at the beginning.

25from saltare = to leap, to dance

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Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

462 Part Three Integration and Control

Physiology Nerve Conduction

Figure 12.15 Saltatory Conduction of a Nerve Signal in a Myelinated Fiber. (a) Ions can be exchanged with the ECF only at the nodes of Ranvier. In the internodes, the nerve signal travels by the rapid diffusion of ions along the inside of the plasma membrane. (b) Action potentials (red) occur only at the nodes, and the nerve signal therefore appears to jump from node to node. Yellow areas indicate refractory (recovering) membrane.

Figure 12.15 Saltatory Conduction of a Nerve Signal in a Myelinated Fiber. (a) Ions can be exchanged with the ECF only at the nodes of Ranvier. In the internodes, the nerve signal travels by the rapid diffusion of ions along the inside of the plasma membrane. (b) Action potentials (red) occur only at the nodes, and the nerve signal therefore appears to jump from node to node. Yellow areas indicate refractory (recovering) membrane.

Before You Go On

Answer the following questions to test your understanding of the preceding section:

  1. What causes K+ to diffuse out of a resting cell? What attracts it into the cell?
  2. What happens to Na+ when a neuron is stimulated on its dendrite? Why does the movement of Na raise the voltage on the plasma membrane?
  3. What does it mean to say a local potential is graded, decremental, and reversible?
  4. How does the plasma membrane at the trigger zone differ from that on the soma? How does it resemble the membrane at a node of Ranvier?
  5. What makes an action potential rise to +35 mV? What makes it drop again after this peak?
  6. List four ways in which an action potential is different from a local potential.
  7. Explain why myelinated fibers transmit signals much faster than unmyelinated fibers.

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Responses

  • Fre-Swera Yonatan
    Is it true that ions can be exchanged with the ecf only at the nodes?
    7 years ago
  • PROCOPIO
    What does it mean to say a local potential is graded, decremental, and reversible?
    1 year ago

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