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Myelinated axons

  • Schwann cells form sheath around neuron
  • gap = node of Ranvier
  • Internode (myelinated portion)
  • Node is really small compared with length of internode
    • node is less than 1 micron, internode length is about 1 mm
  • Leaky pipe model
    • At node, Hodgkin-Huxley model for membrane
    • Internodes are completely insulated
  • Saltatory hypothesis
    • Current 'jumps' from node to node
  • Experimental tests
    • Use core conductor model to infer everything
    • measure [math]\displaystyle{ I_o(z,t) }[/math] for all z and t.
    • data shows decrease in [math]\displaystyle{ I_o }[/math] along internode so leaky pipe model not true
      • A lot of current is lost along internode
    • Two kinds of derived [math]\displaystyle{ K_m }[/math]
      • Biphasic Hodgkin-Huxley like responses (at nodes)
      • Monophasic outward current at internodes consistent with cable model