Myelination in the PNS

• Myelin Sheath


  many neve fibres particularly those that r long/large in diameter r covered in fatty (protein-lipoid), segmented myelin sheath

  myelin protects + electrically insulates fibres + it increases the transmission speed of nerve fibres
• Myelin Sheath Formation in PNS


  myelin sheaths in PNS = formed by Schwann cells, which indent to receive an axon + then wrap themselves around it in a jelly roll fashion
• Wrapping of Schwann Cells Around the Axon


  initially the wrapping is loose, but the Schwann cell cyto is gradually squeezed from between the membrane layers

  when the wrapping process is complete, many concentric layers of Schwann cell plasma membrane enclose the axon

  this tight coil of wrapped membranes = myelin sheath + its thickness depends on the no of spirals
• Structure of Myelins Sheath - Nucleus & Cytoplasm


  the nucleus + most of the cyto of the Schwann cell end up as a bulge just external to the myelin sheath

  = outer collar of perinuclear cyto
• Structure of Myelin Sheath - Plasma Membranes


  plasma membranes of myelinating cells contain much less protein than those of most body cells

  channel + carrier proteins r absent, making myelin sheaths v good electrical insulators

  the presence of specific protein mols that interlock to form a molecular Velcro between myelin membranes
• Structure of Myelin Sheaths - Nodes of Ranvier


  adjacent Schwann cells don't touch one another, so there r gaps in the sheath

  = nodes of Ranvier, which occur at regular intervals along a myelinated axon

  axon collaterals can emerge at these gaps