Editing Axon (section)

===Myelination===
[[File:Myelinated neuron.jpg|thumb|left|[[Transmission electron micrograph|TEM]] of a myelinated axon in cross-section.]]
[[File:Myelin sheath (1).svg|thumb|upright|Cross section of an axon: (1) Axon (2) Nucleus
(3) [[Schwann cell]] (4) [[Myelin sheath]] (5) [[Neurilemma]]]]
In the nervous system, axons may be [[myelin]]ated, or unmyelinated. This is the provision of an insulating layer, called a myelin sheath. The myelin membrane is unique in its relatively high lipid to protein ratio.<ref name="Ozgen">{{cite journal |last1=Ozgen |first1=H |last2=Baron |first2=W |last3=Hoekstra |first3=D |last4=Kahya |first4=N |title=Oligodendroglial membrane dynamics in relation to myelin biogenesis. |journal=Cellular and Molecular Life Sciences |date=September 2016 |volume=73 |issue=17 |pages=3291–310 |doi=10.1007/s00018-016-2228-8 |pmid=27141942|pmc=4967101 }}</ref>

In the peripheral nervous system axons are myelinated by [[neuroglia|glial cells]] known as [[Schwann cell]]s. In the central nervous system the myelin sheath is provided by another type of glial cell, the [[oligodendrocyte]]. Schwann cells myelinate a single axon. An oligodendrocyte can myelinate up to 50 axons.<ref name="Sadler">{{cite book|last1=Sadler|first1=T.|title=Langman's medical embryology|url=https://archive.org/details/langmansmedicale00sadl_655|url-access=limited|date=2010|publisher=Lippincott William & Wilkins|location=Philadelphia|isbn=978-0-7817-9069-7|page=[https://archive.org/details/langmansmedicale00sadl_655/page/n311 300]|edition=11th}}</ref>

The composition of myelin is different in the two types. In the CNS the major myelin protein is [[proteolipid protein]], and in the PNS it is [[myelin basic protein]].