myelin

• what is myelin?

  lipid rich sheath that wraps around axons

  • made up of lipids and protein
• where does myelin come from ?

  oligodendrocytes in the CNS

  Schwann cells in the PNS
• where is myelin found ?

  white matter tracts of brain and spinal chord
• function of myelin

  increase conduction velocity
• myelin protein composition

  maintains myelin structure

  maintains integrity of interactions
• myelin evolution

  myelin not exclusive to mammals

  • in vertebrates and invertebrates
  • evolved in different species at different time points
• myelin enwrapping an axon

  oligodendrocytes send out multiple projections - myelin lamellae - that wrap around multiple axons

  
• myelin lamellae

  formed by fusion of inner leaflets of plasma membrane of oligodendrocyte projections

  trapezoid shaped

  gets flatter in the middle - has a compact region and non-compact region

  
• how myelin wraps axons

  as the projection is wrapping round it gets wider

  
• dense and interperiod lines

  dense lines - inner faces of myelin lamella plasma membrane

  interperiod lines - outer faces of plasma membrane

  
• axon regions

  node - has sodium ion channels

  juxtaparanode - has potassium ion channels

  
• myelin wraps wider on the top than on the bottom

  • wraps by going underneath
• conducting along a nonmyelinated axon

  • site of injection of current there is depolarisation in voltage of membrane
  • current dissipates along axon - both directions
  • voltage decreases along axon
  • length constant (λ) determines how far depolarisation spreads
  • λ depends on membrane resistance (rm) and axoplasmic resistance (ri)
• conduction along nonmyelinated axon 2

  rm - due to leak channels in lipid bilayer

  • more channels =smaller resistance

  ri - due to content of axoplasm

  • more electrically nonconductive molecules = larger resistance

  as rm increases, λ increases

  as ri increases, λ decreases
• rm and ri

  -

  
• how to increase spread of voltage

  increase axon diameter - increases membrane resistance

  decreases axoplasmic resistance - less conc in a set area
• axon diameter effects conduction velocity

  Conduction velocity depends on ri

  • conduction velocity increases as ri decreases
  • If we increase the axon diameter, we decrease ri therefore increase the conduction velocity
• conduction velocity increased by MYELIN

  myelin prevents loss of current

  • Myelin increases rm without needing to increase the axon diameter
  • Greater compactness – process more complex information
  • Increases conduction speed
• conduction along myelinated axon

  saltatory conduction - jumps from node to node
• the g ratio

  optimum number of myelin wraps

  g ratio = axon diameter (di) / axon and myelin diameter (do)