What is the covering over nerves fibers made up of living cells?

The covering over nerve fibers made up of living cells is known as the myelin sheath. This sheath is primarily composed of oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system. Myelin serves to insulate the nerve fibers, allowing for faster transmission of electrical signals along the axon. Additionally, it plays a role in protecting and supporting the nerve fibers.