Which cells form the myelin sheath in the CNS and PNS, respectively?

• A. Oligodendrocytes and Schwann cells
• B. Astrocytes and Microglia
• C. Ependymal cells and Oligodendrocytes
• D. Neurons and Astrocytes

Answer: (A)

Glial cells that form the myelin sheath differ between the CNS and PNS. In the CNS, oligodendrocytes extend their processes to wrap around several nearby axons, creating multiple myelin internodes. In the PNS, Schwann cells wrap around a single axon segment to form one myelin internode. This distinction explains why oligodendrocytes are responsible for CNS myelination and Schwann cells for PNS myelination. The myelin sheaths increase conduction velocity through saltatory conduction, reflecting how each system uses the appropriate glial cell type to optimize signaling. Other glial cells listed—astrocytes, microglia, and ependymal cells—have roles in support, immune function, and CSF production, not myelination. Neurons are signaling cells, not myelin-forming glia.