We know that neurons are encapsulated by myelin. But what makes the myelin?

The brain contains two major classes of cells: neurons and glia. Glia are responsible for creating the myelin sheath, as well as having many other functions.

There are different kinds of glia, including Schwann cells, oligodendrocytes, astroctytes, microglia, and more. The Schwann cells and the oligodendrocytes are responsible for the myelin sheath. Schwann cells do so in the peripheral nervous system (PNS;part of nervous system that isn’t the brain or spinal cord, so, effectually, the nerves), while the oligodendrocytes are responsible for myelin in the central nervous system (CNS;brain and spinal cord).

Both Schwann cells and oligodendrocytes produce thin sheets of myelin that wrap many times around an axon. The myelin in the PNS vs the CNS are similar, but still have differences.