Why neurilemma is absent in central nervous system ?

Options

A. Myelin sheath formation absent in CNS.
B. Oligodendrocytes present in CNS in place of Schwann cells
C. No exchange of ions in CNS
D. Always non-medullated nerve fibres are found in CNS.

Answer

### Step-by-Step Solution:

1. **Definition of Neurilemma**:
- Neurilemma is the outermost layer of the myelin sheath surrounding a nerve fiber, which is formed by Schwann cells in the peripheral nervous system.

2. **Location of Neurilemma**:
- Neurilemma is specifically associated with the peripheral nervous system (PNS) where Schwann cells are present.

3. **Absence in Central Nervous System (CNS)**:
- In the central nervous system (CNS), Schwann cells are absent. Instead, the myelin sheath is formed by a different type of glial cell known as oligodendrocytes.

4. **Function of Oligodendrocytes**:
- Oligodendrocytes perform a similar function to Schwann cells but are adapted for the CNS. They can myelinate multiple axons, unlike Schwann cells which myelinate a single axon.

5. **Conclusion**:
- Since Schwann cells are responsible for forming neurilemma in the PNS and they are not present in the CNS, the neurilemma is therefore absent in the central nervous system.

### Final Answer:
Neurilemma is absent in the central nervous system because it is formed by Schwann cells, which are not present in the CNS. Instead, oligodendrocytes are responsible for forming the myelin sheath in the CNS.

---