The myelin sheath that covers many cns axons is formed by

The myelin sheath that covers many cns axons is formed by oligodendrocytes.

Oligodendrocytes are type of neuroglia (non-neural cells found in central nervous system) with protective, trophic and supportive role. Their function is to insulate axons (with myelin sheath), in the central nervous system (CNS) which is, equivalent to the function performed by Schwann cells in the peripheral nervous system (PNS).