Nodes, paranodes, and incisures: from form to function.
The exquisite molecular architecture of myelinated fibers is the basis for saltatory conduction. The nodal axolemma contains high concentrations of voltage-dependent sodium channels as well as the cell adhesion molecules neurofascin and Nr-CAM, all of which are probably linked to the axonal cytoskeleton by ankyrin. At paranodes, the axonal membrane contains paranodin/Caspr, which may be a Ca(2+)-dependent cell adhesion molecule with a heterophilic partner on the apposed glial cell membrane. The juxtaparanodal axonal membrane contains the potassium channels Kv1.1 and Kv1.2, as well as the associated beta 2 subunit, which together may function to dampen re-entrant excitation. The paranodes and incisures of the Schwann cell myelin sheath contain "reflexive" adherens junctions and gap junctions. The adherens junctions are composed of E-cadherin as well as alpha- and beta-catenin, which together probably join the adjacent layers of noncompact myelin together. Reflexive gap junctions, comprising connexin32 and at least one other connexin protein, form a radial pathway for the diffusion of ions and small molecules directly across the myelin sheath.
Scherer
S S
SS
Department of Neurology, University of Pennsylvania Medical Center, Philadelphia 19104, USA. scherer@mail.med.upenn.edu
D000818:Animals; D001369:Axons / Q000502:physiology; D015815:Cell Adhesion Molecules / Q000502:physiology; D017629:Gap Junctions / Q000502:physiology / Q000648:ultrastructure; D006801:Humans; D008562:Membrane Glycoproteins / Q000502:physiology; D009186:Myelin Sheath / Q000502:physiology* / Q000648:ultrastructure*; D009414:Nerve Growth Factors / Q000502:physiology; D019006:Neural Cell Adhesion Molecules / Q000502:physiology; D009431:Neural Conduction; D009479:Neuropeptides / Q000502:physiology; D011901:Ranvier's Nodes / Q000502:physiology / Q000648:ultrastructure
D015815:Cell Adhesion Molecules; D008562:Membrane Glycoproteins; C054305:NFASC protein, human; D009414:Nerve Growth Factors; D019006:Neural Cell Adhesion Molecules; D009479:Neuropeptides