Myelin sheath consists of

Options

A. A layer of cells
B. Fat
C. Protein
D. Alternate fat and protein layers.

Answer

### Step-by-Step Solution:

1. **Understanding Myelin Sheath**:
- The myelin sheath is a protective covering that surrounds myelinated nerve fibers. It plays a crucial role in the efficient transmission of nerve impulses.

2. **Function of Myelin Sheath**:
- The primary function of the myelin sheath is to insulate the nerve fibers, which allows for faster transmission of electrical signals along the nerve. This is achieved through a process called saltatory conduction, where the nerve impulse jumps from one node of Ranvier to another.

3. **Source of Myelin Sheath**:
- In the peripheral nervous system (PNS), the myelin sheath is produced by Schwann cells. In contrast, in the central nervous system (CNS), it is produced by glial cells.

4. **Composition of Myelin Sheath**:
- The myelin sheath is composed of a lipoprotein bilayer, which includes:
- **Phospholipids**: These are a major component of cell membranes and contribute to the structure of the myelin sheath.
- **Cholesterol**: This helps to stabilize the membrane structure and maintain its fluidity.
- **Proteins**: These are essential for the function and stability of the myelin sheath.

5. **Analyzing the Options**:
- The options provided are:
- A) Layer of cells
- B) Fat
- C) Protein
- D) Alternate fat and protein layers
- Based on the composition discussed, the myelin sheath cannot be accurately described as just a layer of cells, only fat, or only protein. Instead, it consists of alternating layers of fat (lipids) and proteins.

6. **Conclusion**:
- Therefore, the correct answer is **D) Alternate fat and protein layers**.