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2 results

  • 6 - Cable Theory and Saltatory Conduction

  • Christopher L.-H. Huang, University of Cambridge
  • Book:
    Keynes & Aidley's Nerve and Muscle
    Published online:
    07 November 2020
    Print publication:
    19 November 2020, pp 73-87

  • Summary

    Action-potential propagation along the length of an axon beyond the regions of initial excitation requires current flow driven by Na+-channel activation to access remote, initially quiescent, regions of nerve. This current, and its effect on membrane potential, varies with membrane resistance and capacitance, and the electrical resistances of the adjacent extracellular and intracellular fluids. These variables quantify the spread of the consequent voltage change with time and distance through the cable equation. This in turn determines action-potential conduction velocity which, in combination with its refractory period, determines the wavelength of this advancing excitation. Conduction velocity in unmyelinated fibres increases with fibre diameter. That in myelinated fibres increases with the reduced electrical capacitance and increased resistance of their surrounding myelin sheath, resulting in a saltatory action potential conduction. Conduction is further modified by the threshold for the initial excitation, in turn dependent on the membrane Na+ channel density.

  • What is myelin?

  • Daniel K. Hartline
  • Journal:
    Neuron Glia Biology / Volume 4 / Issue 2 / May 2008
    Published online by Cambridge University Press:
    08 September 2009, pp. 153-163
    Print publication:
    May 2008

  • The evolution of a character is better appreciated if examples of convergent emergence of the same character are available for comparison. Three instances are known among invertebrates of the evolution of axonal sheaths possessing the functional properties and many of the structural properties of vertebrate myelin. Comparison of these invertebrate myelins raises the question of what structural features must a sheath possess in order to produce the two principal functional characteristics of impulse speed enhancement and energy savings. This essay reviews the features recognized by early workers as pertaining to myelin in vertebrate and invertebrate alike: osmiophilia, negative birefringence and saltatory conduction. It then examines common features revealed by the advent of electron microscopy: multiplicity of lipid membranes, condensation of those membranes, specialized marginal seals, and nodes. Next it examines the robustness of these features as essential components of a speed-enhancing sheath. Features that are not entirely essential for speed enhancement include membrane compaction, spiral wrapping of layers, glial cell involvement, non-active axonal membrane, and even nodes and perinodal sealing. This permissiveness is discussed in relation to the possible evolutionary origin of myelin.