Which cells form the myelin sheath in the CNS?

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

Answer: (B)

Oligodendrocytes form the myelin sheath in the CNS. These glial cells extend their processes to wrap around several nearby axons, creating segmented myelin sheaths with gaps known as nodes of Ranvier. This insulation increases electrical resistance and decreases membrane capacitance, allowing action potentials to jump from node to node and travel much faster—a process called saltatory conduction. Schwann cells perform a similar insulating role in the peripheral nervous system, but each Schwann cell myelinates only a single axon segment. Astrocytes support neuronal function and the blood-brain barrier, while microglia act as immune cells of the CNS. Thus, oligodendrocytes are the CNS-specific myelin-forming cells.