Multiple sclerosis. Light micrograph of nerve fibres showing demyelination due to multiple sclerosis (MS). Demyelination is a loss of the protective sheath of myelin, a fatty substance that normally covers nerve fibres. The loss of myelin is revealed by the gaps seen between the nerve fibres (black). Myelin helps nerves to conduct signals more efficiently. Loss of myelin disrupts the process, causing symptoms ranging from tingling and numbness to unsteadiness, vision and speech defects and paralysis. The cause of MS is unknown, and there is no effective treatment.