text: The internodes are the myelin segments and the gaps between are referred to as nodes.  The size and the spacing of the internodes vary with the fiber diameter in a curvilinear relationship that is optimized for maximal conduction velocity.  The size of the nodes span from 1–2 μm whereas the internodes can be up to (and occasionally even greater than)1.5 millimetres long, depending on the axon diameter and fiber type.
  The structure of the node and the flanking paranodal regions are distinct from the internodes under the compact myelin sheath, but are very similar in CNS and PNS.  The axon is exposed to the extra-cellular environment at the node and is constricted in its diameter.  The decreased axon size reflects a higher packing density of neurofilaments in this region, which are less heavily phosphorylated and are transported more slowly.  Vesicles and other organelles are also increased at the nodes, which suggest that there is a bottleneck of axonal transport in both directions as well as local axonal-glial signaling. 
  When a longitudinal section is made through a myelinating Schwann cell at the node, three distinctive segments are represented: the stereotypic internode, the paranodal region, and the node itself.  In the internodal region, the Schwann cell has an outer collar of cytoplasm, a compact myelin sheath, and inner collar of cytoplasm, and the axolemma.  At the paranodal regions, the paranodal cytoplasm loops contact thickenings of the axolemma to form septate –like junctions.  In the node alone, the axolemma is contacted by several Schwann microvilli and contains a dense cytoskeletal undercoating.