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Illuminations

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

Cátedra de Fisiología, Facultad de Ciencias Veterinarias, Universidad Nacional de La Plata, 60 y 118 (B1900AVW) La Plata, Argentina e-mail: mauriciog{at}fcv.unlp.edu.ar Department of Physiology, Wayne State University School of Medicine, Detroit, MI 48201 E-mail: sdicarlo{at}med.wayne.edu

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.

View larger version (16K): [in this window] [in a new window]   FIG. 1. Length of axon where polarization occurs in unmyelinated and myelinated axons.

	REFERENCES

TOP REFERENCES

 

1. Goodman BE and Waller SB. Propagation of action potentials in myelinated vs. unmyelinated neurons. Advan Physiol Educ 26: 223, 2002.[Free Full Text]
2. Dowling JE. Neurons and Network: An Introduction to Neuroscience. Cambridge, MA: Harvard Univ. Press, 1992, p. 44.

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