## Core Concept
The myelin sheath in the central nervous system (CNS) is crucial for the proper functioning of neurons, facilitating rapid transmission of electrical impulses along the axon. The formation of the myelin sheath involves specific cells and processes. Understanding which cells are responsible for myelination in the CNS is essential.

## Why the Correct Answer is Right
In the CNS, the myelin sheath is formed by **oligodendrocytes**. These are a type of glial cell or support cell in the CNS. A single oligodendrocyte can extend its processes to multiple axons, wrapping them in layers of myelin. This process, called myelination, is vital for enhancing the speed of neuronal conduction.

## Why Each Wrong Option is Incorrect
- **Option A:** Astrocytes are a type of glial cell in the brain and spinal cord but are not responsible for forming the myelin sheath. They provide nutrients to neurons, maintain the extracellular environment, and have other supportive functions.
- **Option B:** Microglia act as the first and main form of active immune defense in the CNS. They are not involved in myelination.
- **Option D:** Schwann cells are responsible for myelination in the peripheral nervous system (PNS), not the CNS. They wrap around the axons in the PNS to form the myelin sheath.

## Clinical Pearl / High-Yield Fact
A key point to remember is the difference in myelination between the CNS and PNS. In the CNS, oligodendrocytes are responsible for myelination, while in the PNS, Schwann cells perform this function. This distinction is crucial for understanding various neurological conditions and their treatments.

## Correct Answer: C. Oligodendrocytes.