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==Structure of Myelinated Axons==
==Structure of Myelinated Axons==


'''a''' - Myelinating glial cells, oligodendrocytes in the central nervous system (CNS) or Schwann cells in the peripheral nervous system (PNS), form the myelin sheath by enwrapping their membrane several times around the axon. Myelin covers the axon at intervals (internodes), leaving bare gaps — the nodes of Ranvier. Oligodendrocytes can myelinate different axons and several internodes per axon, whereas Schwann cells myelinate a single internode in a single axon.  
'''a''' - Myelinating glial cells, oligodendrocytes in the central nervous system (CNS) or Schwann cells in the peripheral nervous system (PNS), form the myelin sheath by enwrapping their membrane several times around the axon.  
* Myelin covers the axon at intervals (internodes), leaving bare gaps — the nodes of Ranvier.  
* Oligodendrocytes can myelinate different axons and several internodes per axon, whereas Schwann cells myelinate a single internode in a single axon.  


'''b''' - Schematic longitudinal cut of a myelinated fibre around the node of Ranvier showing a heminode. The node, paranode, juxtaparanode (JXP) and internode are labelled. The node is contacted by Schwann cell microvilli in the PNS or by processes from perinodal astrocytes in the CNS. Myelinated fibres in the PNS are covered by a basal lamina. The paranodal loops form a septate-like junction (SpJ) with the axon. The juxtaparanodal region resides beneath the compact myelin next to the paranode (PN). The internode extends from the juxtaparanodes and lies under the compact myelin.  
'''b''' - Schematic longitudinal cut of a myelinated fibre around the node of Ranvier showing a heminode.  
* The node, paranode, juxtaparanode (JXP) and internode are labelled.  
* The node is contacted by Schwann cell microvilli in the PNS or by processes from perinodal astrocytes in the CNS.  
* Myelinated fibres in the PNS are covered by a basal lamina.  
* The paranodal loops form a septate-like junction (SpJ) with the axon.  
* The juxtaparanodal region resides beneath the compact myelin next to the paranode (PN).  
* The internode extends from the juxtaparanodes and lies under the compact myelin.  


'''c''' - Schematic cross-section of a myelinated nerve depicting the inner and outer mesaxons (IMA and OMA, respectively).  
'''c''' - Schematic cross-section of a myelinated nerve depicting the inner and outer mesaxons (IMA and OMA, respectively).  


'''d''' -  Drawing of the specializations found along the internodes. A strand composed of paranodal molecules (Caspr, Contactin; red line) flanked by juxtaparanodal proteins (Caspr2, K+ channels and TAG-1; blue lines) extends along the internodal region (the juxtamesaxon) and below the Schmidt–Lanterman incisures (the juxtaincisure). In addition, Nf155 and ezrin–radixin–moesin proteins, as well as connexins 29 and 32 are found at the glial side, opposite these axonal strands.
'''d''' -  Drawing of the specializations found along the internodes.  
 
* A strand composed of paranodal molecules (Caspr, Contactin; red line) flanked by juxtaparanodal proteins (Caspr2, K+ channels and TAG-1; blue lines) extends along the internodal region (the juxtamesaxon) and below the Schmidt–Lanterman incisures (the juxtaincisure).  
* In addition, Nf155 and ezrin–radixin–moesin proteins, as well as connexins 29 and 32 are found at the glial side, opposite these axonal strands.


(text from figure legend)





Revision as of 10:40, 15 October 2012

Structure of Myelinated Axons

a - Myelinating glial cells, oligodendrocytes in the central nervous system (CNS) or Schwann cells in the peripheral nervous system (PNS), form the myelin sheath by enwrapping their membrane several times around the axon.

  • Myelin covers the axon at intervals (internodes), leaving bare gaps — the nodes of Ranvier.
  • Oligodendrocytes can myelinate different axons and several internodes per axon, whereas Schwann cells myelinate a single internode in a single axon.

b - Schematic longitudinal cut of a myelinated fibre around the node of Ranvier showing a heminode.

  • The node, paranode, juxtaparanode (JXP) and internode are labelled.
  • The node is contacted by Schwann cell microvilli in the PNS or by processes from perinodal astrocytes in the CNS.
  • Myelinated fibres in the PNS are covered by a basal lamina.
  • The paranodal loops form a septate-like junction (SpJ) with the axon.
  • The juxtaparanodal region resides beneath the compact myelin next to the paranode (PN).
  • The internode extends from the juxtaparanodes and lies under the compact myelin.

c - Schematic cross-section of a myelinated nerve depicting the inner and outer mesaxons (IMA and OMA, respectively).

d - Drawing of the specializations found along the internodes.

  • A strand composed of paranodal molecules (Caspr, Contactin; red line) flanked by juxtaparanodal proteins (Caspr2, K+ channels and TAG-1; blue lines) extends along the internodal region (the juxtamesaxon) and below the Schmidt–Lanterman incisures (the juxtaincisure).
  • In addition, Nf155 and ezrin–radixin–moesin proteins, as well as connexins 29 and 32 are found at the glial side, opposite these axonal strands.

(text from figure legend)


Links: Medicine - Neural Histology | Neural System Development | Postnatal Neural


Reference

<pubmed>14682359</pubmed>| Nat Rev Neurosci.


Reprinted by permission from Macmillan Publishers Ltd: Nature Reviews Neuroscience 4, 968-980 (December 2003) | doi:10.1038/nrn1253, copyright (2003)

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