Myelin sheath is formed from

Options

A. Nerve cells
B. Glial cells
C. Schwann cells
D. None of the above.

Answer

### Step-by-Step Solution:

1. **Understanding Myelin Sheath**: The myelin sheath is a protective covering that surrounds nerve fibers (axons). It is crucial for the proper functioning of the nervous system as it increases the speed of electrical impulses along the axon.

2. **Composition of Myelin Sheath**: The myelin sheath is primarily composed of a modified plasma membrane. This means that it is not just a simple layer but an extended and specialized form of the cell membrane.

3. **Source of Myelin Sheath**: The myelin sheath is formed by specific types of cells. In the peripheral nervous system (PNS), the myelin sheath is produced by Schwann cells. In contrast, in the central nervous system (CNS), oligodendrocytes are responsible for myelin formation.

4. **Evaluating Options**:
- **Option 1: Nerve Cells** - This option is incorrect because nerve cells (neurons) do not directly form the myelin sheath.
- **Option 2: Glial Cells** - While glial cells are involved in supporting neurons, this option is too broad and not specific to the formation of the myelin sheath.
- **Option 3: Schwann Cells** - This is the correct option as Schwann cells are specifically responsible for forming the myelin sheath in the peripheral nervous system.
- **Option 4: None of the Above** - This is incorrect as we have identified Schwann cells as the correct answer.

5. **Conclusion**: The myelin sheath is formed from Schwann cells in the peripheral nervous system.

### Final Answer:
The myelin sheath is formed from **Schwann cells**.

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