Demyelinated nerves. Light micrograph of a section through nerve fibres showing demyelination. Demyelination is a loss of the protective sheath of myelin, a fatty substance that normally covers nerve fibres and 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. Demyelination can be caused by a number of conditions, including multiple sclerosis.