Myelin forms a sheath around neurons in the brain which helps to increase the speed at which information, in the form of electrical impulses, can travel. Without myelin, neurons tend to be larger and require more energy. For example, the unmyelinated giant axon of the squid has a 500 mm diameter and requires 5,000 times as much energy as a myelinated axon in a frog which is only 12 mm in diameter . When myelin is altered or damaged, motor and sensory function can be affected; multiple sclerosis is one of the best-known diseases in which demyelination plays a role. Currently there is not a cure for demyelinating diseases; instead treatments focus on reducing symptoms .
Myelination begins as early as the fifth fetal month or towards the end of the second trimester. Rapid myelination continues till around the second year of life and then continues more gradually into young adulthood . Myelin is composed of proteins and lipids; a few of its major constituents are myelin basic proteins (MBP) and myelin associated glycoprotein (MAG) which are shown in figure 14.
MBP: accounts for approximately 30% of myelin proteins
MAG: accounts for approximately 1% of myelin, but it may have a role in the initiation of myelination
Adapted from "Magnetic resonance imaging of myelin" by Laule, C., Vavasour, I.M., Kolind, S.H. et al., 2007, GLIA, 4, p. 460. Copyright 2007 by Springer New York