Myelin is produced by specialized glial cells that wrap around neuronal axons, forming a protective and insulating sheath crucial for rapid nerve impulse transmission. This process differs slightly between the central nervous system (CNS) and the peripheral nervous system (PNS).
Myelin Production in the PNS: Schwann Cells
In the PNS, myelin is formed by Schwann cells. The process unfolds as follows:
- Envelopment: A Schwann cell initially envelops a portion of the axon.
- Spiral Wrapping: The Schwann cell's plasma membrane begins to spiral around the axon. This is the core of the myelination process.
- Myelin Sheath Formation: With continued wrapping, multiple layers of Schwann cell membrane accumulate around the axon, forming the myelin sheath. Think of it like rolling up a sleeping bag tightly around something.
- Cytoplasm Displacement: During this process, the cytoplasm of the Schwann cell is squeezed out from between the membrane layers, compacting the myelin sheath.
- Nodes of Ranvier: Each Schwann cell myelinates only a small segment of the axon, leaving small gaps called Nodes of Ranvier between adjacent Schwann cells. These nodes are crucial for saltatory conduction, the "jumping" of action potentials that speeds up nerve impulse transmission.
Myelin Production in the CNS: Oligodendrocytes
In the CNS, myelin is formed by oligodendrocytes. The key difference is that a single oligodendrocyte can myelinate multiple axons or multiple segments of the same axon. The process includes:
- Extension of Processes: Oligodendrocytes extend multiple sheet-like processes towards different axons.
- Axon Contact and Wrapping: Each process contacts an axon and begins to spiral around it, similar to the Schwann cell mechanism. The innermost glial process makes contact with the axon.
- Myelin Sheath Formation: As the oligodendrocyte processes continue to wrap, multiple layers of membrane form the myelin sheath. Cytoplasm gets squeezed out from all but the innermost and outermost layers.
- Nodes of Ranvier: Like in the PNS, gaps (Nodes of Ranvier) exist between myelinated segments, allowing for saltatory conduction.
Key Similarities and Differences
| Feature | PNS (Schwann Cells) | CNS (Oligodendrocytes) |
|---|---|---|
| Myelinating Cell | Schwann cell | Oligodendrocyte |
| Axons Myelinated | One segment of one axon | Multiple axons or segments of one axon |
| Location | Peripheral nervous system | Central nervous system |
| Mechanism | Spiral wrapping of cell around axon | Extension of processes to wrap axons |
Importance of Myelin
Myelin is essential for the efficient functioning of the nervous system. It:
- Increases the speed of nerve impulse transmission. Saltatory conduction allows action potentials to "jump" between Nodes of Ranvier, drastically increasing transmission speed compared to unmyelinated axons.
- Provides insulation for axons. This prevents the signal from degrading as it travels down the axon.
- Supports the long-term health and survival of axons.
Damage to myelin, as seen in diseases like multiple sclerosis, can lead to significant neurological deficits.
