Which statement is correct about nerve fibres?

Options

A. All nerve fibres of CNS are enclosed by Schwann cells.
B. Only myelinated nerve fibres of PNS are enclosed by Schwann cells.
C. All nerve fibres of PNS are inclosed by Schwann cells.
D. Only myelinated fibres of CNS are enclosed by schwann cells.

Answer

To determine which statement is correct about nerve fibers, let's analyze the provided options step by step:

### Step 1: Understand the Types of Nerve Fibers
Nerve fibers can be classified into two main categories: myelinated and unmyelinated fibers. Myelinated fibers have a protective myelin sheath that increases the speed of nerve impulse transmission, while unmyelinated fibers do not.

### Step 2: Analyze Each Statement
1. **First Statement**: "All nerve fibers of the central nervous system are enclosed by the Schwann cells."
- This statement is incorrect. Schwann cells are primarily associated with the peripheral nervous system (PNS). In the central nervous system (CNS), oligodendrocytes are the cells that form the myelin sheath around nerve fibers.

2. **Second Statement**: "Only myelinated nerve fibers of the peripheral nervous system are enclosed by the Schwann cells."
- This statement is also incorrect. Both myelinated and unmyelinated nerve fibers in the PNS are supported by Schwann cells. Schwann cells can wrap around myelinated fibers to form the myelin sheath and also provide support to unmyelinated fibers.

3. **Third Statement**: "All nerve fibers of the peripheral nervous system are enclosed by the Schwann cells."
- This statement is correct. In the PNS, Schwann cells are responsible for the myelination of myelinated fibers and also provide support to unmyelinated fibers.

4. **Fourth Statement**: "Only myelinated fibers of the central nervous system are enclosed by the Schwann cells."
- This statement is incorrect. As mentioned earlier, Schwann cells are not present in the CNS; oligodendrocytes are the cells responsible for myelination in the CNS.

### Conclusion
The correct statement about nerve fibers is the third option: "All nerve fibers of the peripheral nervous system are enclosed by the Schwann cells."