Conduction in a Myelinated Axon

The myelin sheath provides insulation for the axon, preventing movements of Na+ and K+ through the membrane. If the myelin sheath were continuous, therefore, action potentials could not be produced. The myelin thus has interruptions—the nodes of Ranvier, as previously described.

Because the cable properties of axons can conduct depolarizations only over a very short distance (1 to 2 mm), the nodes of Ranvier cannot be separated by more than this distance. Studies have shown that Na+ channels are highly concentrated at the nodes (estimated at 10,000 per square micrometer) and almost absent in the regions of axon membrane between the nodes. Action potentials, therefore, occur only at the nodes of Ranvier (fig. 7.18) and seem to "leap" from node to node—a process called saltatory conduction (saltario = leap). The leaping is, of course, just a metaphor; the action potential at one node depolarizes the membrane at the next node to threshold, so that a new action potential is produced at the next node of Ranvier.

Since the cablelike spread of depolarization between the nodes is very fast and fewer action potentials need to be produced per given length of axon, saltatory conduction allows a faster rate of conduction than is possible in an unmyelinated fiber. Conduction rates in the human nervous system vary from 1.0 m/sec—in thin, unmyelinated fibers that mediate slow, visceral responses—to faster than 100 m/sec (225 miles per hour)—in thick, myelinated fibers involved in quick stretch reflexes in skeletal muscles (table 7.4).

In summary, the speed of action potential conduction is increased by (1) increased diameter of the axon, because this

The Nervous System: Neurons and Synapses

Table 7.4

Conduction Velocities

and Functions of Mammalian Nerves

of Different Diameters

Diameter (|m)

Conduction Velocity (m/sec)

Examples of Functions Served



Sensory: muscle position



Somatic motor fibers



Sensory: touch, pressure



Sensory: pain, temperature



Autonomic fibers to ganglia



Autonomic fibers to smooth and cardiac muscles

reduces the resistance to the spread of charges by cable properties; and (2) myelination, because the myelin sheath results in saltatory conduction of action potentials. These methods of affecting conduction speed are generally combined in the nervous system: the thinnest axons tend to be unmyelinated and the thickest tend to be myelinated.

Test Yourself Before You Continue

1. Define the terms depolarization and repolarization, and illustrate these processes graphically.

2. Describe how the permeability of the axon membrane to Na+ and K+ is regulated and how changes in permeability to these ions affect the membrane potential.

3. Describe how gating of Na+ and K+ in the axon membrane results in the production of an action potential.

4. Explain the all-or-none law of action potentials and describe the effect of increased stimulus strength on action potential production. How do the refractory periods affect the frequency of action potential production?

5. Describe how action potentials are conducted by unmyelinated nerve fibers. Why is saltatory conduction in myelinated fibers more rapid?

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