6.021/Notes/2006-11-15

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