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Oligodendrocyte nerve cells, illustration. Oligodendrocytes form the myelin sheath around neurons (nerve cells) in the central nervous system (CNS). The myelin sheath insulates the nerve axon, allowing for faster transmission of electrical impulses. One oligodendrocyte may be responsible for the myelination of up to 50 nerve fibres. (KEYSTONE/SCIENCE PHOTO LIBRARY/JUAN GAERTNER/SCIENCE PHOTO LIBRARY)