File:Structure of myelinated axons 01.jpg

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 (nodes of Ranvier).
• Oligodendrocytes can myelinate different axons and several internodes per axon.
• 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)

http://www.nature.com/nrn/journal/v4/n12/fig_tab/nrn1253_F1.html