Neurilemma, also known as neurolemma, Schwann's sheath, or sheath of Schwann, is the outermost layer of a nerve fiber in the peripheral nervous system. It's essentially the outermost nucleated cytoplasmic layer of Schwann cells (also called neurilemmocytes) that surrounds the axon of a neuron.
Understanding the Neurilemma
The neurilemma plays a crucial role in nerve regeneration. Unlike the central nervous system (CNS), where nerve regeneration is limited, the peripheral nervous system (PNS) demonstrates significant regenerative capacity, largely due to the neurilemma. The Schwann cells within the neurilemma guide the regrowth of damaged axons.
Key Features of the Neurilemma:
- Location: Peripheral Nervous System (PNS)
- Composition: Outermost layer of Schwann cells.
- Function: Supports nerve regeneration; separates layers of myelin in myelinated fibers.
- Difference from Myelin Sheath: While the myelin sheath provides insulation, the neurilemma is the outer protective layer surrounding the myelin (in myelinated fibers) and the axon itself (in unmyelinated fibers). The myelin sheath is not present in all nerve fibers; the neurilemma is always present in PNS fibers.
Comparison with Myelin Sheath
| Feature | Myelin Sheath | Neurilemma |
|---|---|---|
| Location | Both CNS and PNS (but formed by different cells) | PNS only |
| Function | Insulation, faster nerve impulse conduction | Support, regeneration |
| Cellular Origin | Oligodendrocytes (CNS), Schwann cells (PNS) | Schwann cells |
| Presence | Present in myelinated axons | Present in both myelinated and unmyelinated axons |
This outer layer, the neurilemma, is vital for the repair and regrowth of damaged nerves in the peripheral nervous system. In contrast, the absence of a neurilemma in the central nervous system contributes to the limited regenerative capacity of CNS neurons.
