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 that surrounds the axons of neurons in the peripheral nervous system (PNS). It is formed by Schwann cells and plays a crucial role in the regeneration of damaged nerves.

2. **Presence in Peripheral Nervous System**: In the PNS, the neurilemma is present because Schwann cells wrap around the axons, creating a protective layer that aids in nerve regeneration and repair.

3. **Absence in Central Nervous System**: In contrast, the central nervous system (CNS), which includes the brain and spinal cord, does not have neurilemma. This is because the myelin sheath in the CNS is formed by oligodendrocytes, which do not produce a neurilemma.

4. **Role of Oligodendrocytes**: Oligodendrocytes can myelinate multiple axons at once, but they do not form a neurilemma. This is a key difference between the myelination process in the CNS and PNS.

5. **Implications of Absence**: The absence of neurilemma in the CNS means that the regeneration of damaged neurons is limited compared to the PNS, where the presence of neurilemma facilitates repair.

### Summary:
Neurilemma is absent in the central nervous system because the myelin sheath in the CNS is formed by oligodendrocytes, which do not create a neurilemma, unlike Schwann cells in the peripheral nervous system.

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