MYELINATED VS. UNMYELINATED NERVE CONDUCTION: A NOVEL WAY OF UNDERSTANDING THE MECHANISMS

Mauricio J. Giuliodori, Stephen E. DiCarlo

The concepts and physiological significance of saltatory nerve conduction are often difficult for students to grasp. Most physiology textbooks contain a variation of: “… the action potential leaps from node to node along the axon. …”A clever analogy of these events has recently been published (1); however, the mechanism mediating saltatory nerve conduction was not included. To help students understand the mechanism mediating saltatory nerve conduction, we emphasize that action potential propagation depends on the activation of voltage-gated sodium channels. We point out that unmyelinated axons have voltage-gated sodium channels along the entire length of the membrane. In contrast, myelinated axons have voltage-gated sodium channels only in the nodal spaces. Nodal spaces (nodes of Ranvier) are unmyelinated spaces ∼2 μm long. The unmyelinated spaces are located at ∼1-mm intervals along the axonal surface (internodal spaces: myelinated wraps) (2). Action potential propagation along unmyelinated axons requires activation of voltage-gated sodium channels along the entire length of the axon. In sharp contrast, action potential propagation along myelinated axons requires activation of voltage-gated sodium channels only in the nodal spaces. With this understanding, the students realize that action potential propagation is much faster along myelinated axons. To further emphasize this point, we provide the following example. Consider a myelinated axon 1,500,000 μm long. Only 0.2% of the myelinated axon (2,994 μm) contains nodes of Ranvier where depolarization occurs. Similarly, a myelinated axon with a total surface area of 1,178,100 μm2 has only 2,352 μm2 of membrane (0.2%) where depolarization occurs (Table 1). Assuming equal time constants for activation of voltage-gated sodium channels along myelinated and unmyelinated axons, the myelin sheath reduces the length and surface area where depolarization occurs and increases action potential propagation velocity. This concept is illustrated in Fig. 1.

FIG. 1.

Length of axon where polarization occurs in unmyelinated and myelinated axons.

View this table:
Table 1.

Axon length and surface area divided into internodal and nodal spaces

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