Once axons have been formed through differentiation, it is important to optimize them for use. This is because the axons have a very important role in transmitting signals when the brain is fully developed. To make this signal transfer faster, a layer of myelin is formed around the axons.

Myelin is a fatty white substance that acts as a kind of insulation material. This not only allows signals to be transmitted more quickly, but it also ensures that the signal only reaches the synapse at the end of the axon. Without myelin, halfway down the axon the signal could already jump to a cell for which the signal is not intended.

Myelin in the brain is formed by oligodendrocytes. These are cells that wrap around axons, then turn into myelin. An oligodendrocyte can supply about a dozen axons with myelin.

Myelination is a process that takes many years, from birth to an age of 18. In this process, the myelin at the motor and sensory areas develops first. This occurs when a child is 3 or 4 years old. The association areas in the posterior and anterior parts of the brain are the last to be supplied with myelin. As we age, the myelin in the brain again slowly diminishes, so older people often have difficulty with actions that were once the most normal.

Multiple sclerosis (MS) is a condition in which the myelin is affected and eventually disappears. This process creates holes in the myelin. These holes are filled with ordinary connective tissue, a substance with no insulating properties. There is hardening (sclerosis) of the axons in this case. Due to this hardening, the transmission of signals becomes increasingly difficult, and eventually impossible.

Author: Myrthe Princen (translated by Melanie Smekal)
Image: Marcel Loeffen