Which glial cell wraps axons in the CNS with a myelin sheath?

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

Answer: (D)

Oligodendrocytes are the glial cells responsible for myelinating axons in the central nervous system. Each oligodendrocyte extends several processes that wrap around parts of different axons, creating multiple layers of myelin that insulate those segments. This insulation speeds up electrical signaling by increasing membrane resistance and reducing capacitance, enabling saltatory conduction at the gaps between myelinated segments called nodes of Ranvier. In the CNS, one oligodendrocyte can myelinate portions of many axons, whereas in the peripheral nervous system, Schwann cells wrap myelin around a single axon segment. Astrocytes, microglia, and ependymal cells serve other roles: astrocytes support neurons and help maintain the blood–brain barrier, microglia act as immune cells, and ependymal cells line the ventricles and participate in CSF production and flow.