Lecture Notes: Function of the Myelin Sheath in Transmitting Action Potentials

Introduction

• Focus: Function of the myelin sheath in nerve impulse transmission.
• Myelin Sheath: Previously discussed as a structure formed by Schwann cells wrapping around the axon.
• Nodes of Ranvier: Gaps in the myelin sheath, unmyelinated parts of the axon.

Structure and Function

• Myelin Sheath: Provides electrical insulation; made of Schwann cells.
  • Reduces permeability to ions due to predominantly non-polar lipid composition.
• Sodium Ion Channels: Concentration varies along the axon.
  • Myelinated Sections: Fewer sodium ion channels.
  • Nodes of Ranvier: More sodium ion channels.

Electrical Insulation

• Myelin sheath acts as an insulator, preventing ion diffusion.
• Lipids in the sheath repel ions, hindering their movement across the membrane.

Implications of Myelination

• Myelinated regions are less permeable to ions; nodes of Ranvier more permeable.
• Insulation by myelin critical for speeding up action potentials.

Action Potential Propagation

• Unmyelinated Axons: Continuous depolarization along axon sections.
• Myelinated Axons: Action potentials "jump" from node to node (saltatory conduction).

Saltatory Conduction

• Definition: Appearance of action potentials "jumping" between nodes of Ranvier.
• Mechanism:
  • Local circuits in myelinated axons bypass myelinated sections and activate nodes.
  • Faster transmission as depolarization skips insulated areas.

Benefits of Saltatory Conduction

• Speed: Myelinated axons conduct impulses more rapidly than unmyelinated ones.
  • At the same time, myelinated axons reach the end of the axon faster.

Conclusion

• Key Function of Myelin Sheath: Speed up action potential by insulating axons, causing potential to jump along nodes of Ranvier.
• Saltatory Conduction: Critical for rapid nerve impulse transmission.

Summary

• Myelin sheath provides electrical insulation, facilitating rapid transmission of nerve impulses through saltatory conduction.