Myelin Sheath: How it Speeds Up Nerve Impulse Transmission

The myelin sheath does not transmit impulses between neurons. Instead, it insulates axons to dramatically increase conduction velocity of action potentials within a single neuron.

Mechanism: Saltatory Conduction

1. Myelin (lipid‑rich layers from oligodendrocytes in CNS, Schwann cells in PNS) wraps the axon segmentally.
2. Nodes of Ranvier are short (~1 µm), unmyelinated gaps rich in voltage‑gated Na⁺/K⁺ channels.
3. Depolarization occurs only at nodes; the current "jumps" from node to node.
4. Result: Conduction speed increases 10–100× (up to 120 m/s) vs. unmyelinated fibers (0.5–2 m/s), with less energy expenditure.

Key Functions of Myelin

• Insulation: High membrane resistance, low capacitance → minimal ion leakage.
• Energy efficiency: Fewer Na⁺/K⁺‑ATPase cycles needed per impulse.
• Structural support: Maintains axon integrity; Schwann cells aid regeneration after injury.

Clinical Correlates

• Demyelinating diseases: Multiple sclerosis (CNS), Guillain‑Barré syndrome (PNS) → slowed/blocked conduction, neurological deficits.
• Charcot‑Marie‑Tooth disease: Hereditary myelin defects.
• Developmental: Myelination continues into early adulthood; correlates with cognitive maturation.

In essence, myelin transforms the axon into a high‑speed, low‑loss transmission line—enabling the rapid, coordinated signaling essential for complex nervous system function.