AAMC FL Practice Exam

Which cells form the myelin sheath in the central nervous system (CNS)?

1. Microglia

2. Astrocytes

3. Schwann cells

4. Oligodendrocytes

The correct answer is: Oligodendrocytes

In the central nervous system (CNS), the formation of the myelin sheath is carried out by oligodendrocytes. These specialized glial cells extend their cellular processes to wrap around the axons of neurons, providing insulation that enhances the speed of electrical signal transmission along the nerve fibers. The myelin sheath created by oligodendrocytes consists of multiple layers of membrane, which is critical for efficient nerve conduction. In contrast, other cell types mentioned have different roles in the nervous system. Microglia function as the primary immune cells of the CNS, responsible for clearing debris and responding to injury. Astrocytes provide structural support and nourishment to neurons while also regulating the extracellular environment and contributing to the blood-brain barrier. Schwann cells, while also responsible for myelination, specifically do so in the peripheral nervous system (PNS), wrapping around peripheral axons to facilitate nerve signal conduction. Therefore, the key function of oligodendrocytes in forming the myelin sheath within the CNS distinguishes them as the correct choice in this context.