What produces the myelin sheath in the CNS?

• A. Oligodendrocytes
• B. Schwann cells
• C. Astrocytes
• D. Microglia

Answer: (A)

In the CNS, myelin is produced by oligodendrocytes. These glial cells extend multiple processes that wrap around several axons, creating insulated segments (internodes) with gaps called nodes of Ranvier. This wrapping speeds nerve conduction through saltatory conduction, and unlike in the peripheral nervous system—where Schwann cells wrap a single axon—one oligodendrocyte can myelinate many different axons. Astrocytes and microglia have other roles in support and immune defense, but they do not form the myelin sheath. The myelin itself is rich in lipids, providing the insulating layer that greatly increases conduction velocity. In diseases that damage oligodendrocytes, such as multiple sclerosis, myelin loss slows or blocks CNS signaling.