Multiple sclerosis is a condition where the myelin sheath hardens; disrupts the flow of information through the neurons. Myelin sheath is a layer of fatty cells which helps in the transmission of nerves impulses along the axon by insulating it. It is an outgrowth of a type of glial cell which is formed by a process called myelination or myelinogenesis.
Therefore, the correct answer is Myelin sheath.
