15.03 Myelin

Structure and Function:

• Myelin:
  • Definition: Insulating material around the axons of many neurones.
  • Composition: Made of multiple layers of Schwann cell membranes, rich in phospholipids (lipid-rich).
  • Purpose: Prevents ion and water flow between tissue fluid and the axon, enhancing electrical insulation.
• Schwann Cells:
  • Wrap around the axon to form the myelin sheath.
  • Myelination Process: Schwann cells spiral around the axon, creating a multi-layered lipid sheath.

• Nodes of Ranvier:
  • Location: Small gaps (2–3 μm) between Schwann cells, occurring every 1–3 mm along a myelinated axon.
  • Function: Exposed areas allow ion exchange, facilitating saltatory conduction (rapid impulse transmission by “jumping” between nodes).

FIGURE: Action potential propagation in myelinated neurons is faster than in unmyelinated neurons because of saltatory conduction.

Myelination in the Nervous System:

FIGURE: Transmission electron micrograph of a cross-section of a myelinated PNS axon (the dark rings above), generated at the Electron Microscope.

• Myelinated Axons:
  • Visible in cross-sections as dark rings around axons.
  • Enable faster transmission of nerve impulses due to insulation.
• Unmyelinated Axons:
  • Lack the dark myelin rings.
  • Prevalence: Approximately two-thirds of motor and sensory neurones are unmyelinated, leading to slower impulse conduction.

Advantages of Myelination:

• Speed: Myelin greatly increases the speed of impulse transmission along axons.
• Energy Efficiency: Reduces the metabolic cost of maintaining ion gradients along the entire axon.

Key Terms:

• Node of Ranvier: Gap between myelinated sections where impulses jump, enhancing transmission speed.
• Myelin Sheath: Multi-layered Schwann cell covering, mostly lipid, that insulates the axon.