Impulse Conduction

An unmyelinated axon conducts an impulse over its entire surface. A myelinated axon functions differently. Myelin contains a high proportion of lipid that excludes water and water-soluble substances. Thus, myelin serves as an insulator and prevents almost all flow of ions through the membrane that it encloses.

It might seem that the myelin sheath would prevent conduction of a nerve impulse, and this would be true if the sheath were continuous. However, nodes of Ranvier

Region of action potential I-'-1

Region of action potential I-'-1

Figure 10.15

Figure 10.15

(a) An action potential in one region stimulates the adjacent region, and (b and c) a wave of action potentials (a nerve impulse) moves along the axon. (Negative ions not shown.)

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