Myelin and propagation of the action potential

Creation of the myelin sheath was an extremely important evolutionary step, which dramatically boosted the velocity of nerve impulse propagation, which was critically important for increase in the animal size. The speed of conduction of a given axon is directly and strictly dependent on axon diameter, whereby larger axons conduct faster than smaller diameter axons (Figure 8.9); this is a function of the relationship between the low resistance of the axoplasm and the high resistance and capacitance of the axolemma. The myelin sheath comprises multiple layers of cell membrane (see Chapter 8.8.1), and therefore has ultrahigh resistance and capacitance, enabling smaller diameter axons to conduct impulses much faster than unmyelinated axons of equivalent diameter (Figure 8.9). In nonmyelinated axons the action potential conductance velocity is proportional to half of the axon diameter (v a D/2), whereas in myelinated ones the speed of impulse propagation is proportional to the axon diameter (Figure 8.9). The maximal nerve impulse conduction velocity, characteristic for thick (~20-30 ^m in diameter) mammalian myelinated axons lies around 80-120 m/s; to achieve the comparable AP propagation speed in nonmyelinated nerves, the axon diameter

Axon Diameter Nervous Impulses

Figure 8.9 Relationship between conduction velocity and axon diameter. Larger axons conduct faster than smaller axons, and the insulation of the myelin sheath enables smaller diameter axons to conduct impulses much faster than unmyelinated axons of equivalent diameter. To achieve the fastest conduction velocities observed in myelinated axons, unmyelinated axons would need to be as large as 500-1000 |xm (which is the case in invertebrates). The miniaturization provided by myelin allows the development of highly complex nervous systems seen at their peak in mammals.

(Modified from Waxman SG, Bangalore L (2005) Myelin function and saltatory conduction, In: Neuroglia, Kettenmann H & Ransom BR, Eds, OUP, 2005, p. 274)

Figure 8.9 Relationship between conduction velocity and axon diameter. Larger axons conduct faster than smaller axons, and the insulation of the myelin sheath enables smaller diameter axons to conduct impulses much faster than unmyelinated axons of equivalent diameter. To achieve the fastest conduction velocities observed in myelinated axons, unmyelinated axons would need to be as large as 500-1000 |xm (which is the case in invertebrates). The miniaturization provided by myelin allows the development of highly complex nervous systems seen at their peak in mammals.

(Modified from Waxman SG, Bangalore L (2005) Myelin function and saltatory conduction, In: Neuroglia, Kettenmann H & Ransom BR, Eds, OUP, 2005, p. 274)

should be as large as 500-1000 ^m. Quite obviously such a thickness would be incompatible with a necessity to pack hundreds of thousands of axons into a single nerve in higher animals. Thus, myelin provides for miniaturization of the nervous system; invertebrates such as the squid, for example, require giant axons (1000 ^m diameter) to mediate rapid events such as the contraction of their mantle during the escape reflex. The thinnest myelinated fibres have a diameter ~0.2 ^m, and there is very little difference in impulse propagation velocity in nonmyelinated and myelinated fibres at this diameter.

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Responses

  • cindy english
    Why do thicker axons conduct faster?
    5 years ago
  • kaj
    What is the role of myelin sheath in action potential propagation?
    5 years ago
  • Sisko
    Why does the myelin sheath make the action potential propagate faster?
    4 years ago
  • FIORI
    What is the role of the schwann cells in the propagation of the nerve action potential?
    4 years ago

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