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In page Hereditary spastic paraplegia:

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Axons in the central and peripheral nervous system are coated with an insulation, the myelin layer, to increase the speed of action potential propagation. Abnormal myelination in the CNS is detected in some forms of hsp HSP.[1] Several genes were linked to myelin malformation, namely PLP1, GFC2 and FA2H.[2] The mutations alter myelin composition, thickness and integrity.[citation needed]