Brain in multiple sclerosis. Section through the brain of a patient with multiple sclerosis (MS), showing plaques of demyelinated nerves in the cortex. MS is an inflammatory disease in which demyelination occurs in the nerves of the brain and spinal cord. 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.