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Myelin

Neurones that need to transmit electrical signals quickly are sheathed by a fatty substance called myelin. Myelin acts as an electrical insulator, and signals travel 20 times faster when it is present. In the disease multiple sclerosis, the myelin around the axons of some nerves gradually breaks down, so that the nerves can no longer efficiently carry electric signals between the brain and body.

Electron micrograph of nerve fibres, some surrounded by dark myelin sheaths.
Electron micrograph of nerve fibres, some surrounded by dark myelin sheaths.
Mike Kayser/Wellcome Photo Library

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