A neuron is a microscopic structure composed of three major parts, namely, cell body, dendrites and axon. There are two types of axons, namely, myelinated, and nonmyelinated. Which of the following is true for nerve ?
i. The myelinated nerve fibres are enveloped with Schwann cells
ii. Schwann cells form a myelin sheath around the axom
iii. The gaps between two adjacent myelin sheaths are called nodes of Ranvier.
iv.Myelinated nerve fibres are found in autonomous and the somatic neural systems
v. Unmyelinated nerve fibre is enclosed by a Schwann cell that does not form a myelin sheath around the axon.
vi. Unmyelinated nerve fibre is commonly found in spinal and cranial nerves

Options

A. i, ii and iii
B. i, ii, iii and v
C. i, ii, iii iv and vi
D. All of these

Answer

To solve the question regarding the characteristics of myelinated and unmyelinated nerve fibers, we will evaluate each statement provided and determine which ones are true.

1. **Statement i**: "The myelinated nerve fibers are enveloped with Schwann cells."
- **Evaluation**: This statement is true. Myelinated nerve fibers are indeed enveloped by Schwann cells, which wrap around the axon to form the myelin sheath.

2. **Statement ii**: "Schwann cells form a myelin sheath around the axon."
- **Evaluation**: This statement is also true. Schwann cells are responsible for forming the myelin sheath around the axons in the peripheral nervous system.

3. **Statement iii**: "The gaps between two adjacent myelin sheaths are called nodes of Ranvier."
- **Evaluation**: This statement is true. The nodes of Ranvier are the gaps that occur between adjacent myelin sheaths along a myelinated axon.

4. **Statement iv**: "Myelinated nerve fibers are found in autonomous and the somatic neural systems."
- **Evaluation**: This statement is partially true. Myelinated nerve fibers are found in both the autonomic nervous system (ANS) and the somatic nervous system (SNS), but the statement may imply that all fibers in these systems are myelinated, which is not accurate as there are also unmyelinated fibers.

5. **Statement v**: "Unmyelinated nerve fiber is enclosed by a Schwann cell that does not form a myelin sheath around the axon."
- **Evaluation**: This statement is true. Unmyelinated fibers are indeed enclosed by Schwann cells, but these cells do not form a myelin sheath around the axon.

6. **Statement vi**: "Unmyelinated nerve fiber is commonly found in spinal and cranial nerves."
- **Evaluation**: This statement is misleading. While unmyelinated fibers can be found in the peripheral nervous system, most fibers in spinal and cranial nerves are myelinated.

### Conclusion:
Based on the evaluations:
- True statements: i, ii, iii, v
- Partially true statement: iv
- False statement: vi

Thus, the correct answer includes statements i, ii, iii, and v.

### Final Answer:
The true statements regarding nerve fibers are:
- i, ii, iii, and v.