Myelin Sheath Formation: CNS vs. PNS Glial Cells, Function, and Clinical Correlates

Myelinating Glia: Central vs. Peripheral Nervous System

Feature	Central Nervous System (CNS)	Peripheral Nervous System (PNS)
Myelinating cell	Oligodendrocyte	Schwann cell
Axons myelinated per cell	Multiple (up to 50)	One axon segment per cell
Myelin membrane origin	Oligodendrocyte process extension	Schwann cell plasma membrane
Regeneration capacity	Limited (no basal lamina)	Robust (basal lamina guides regrowth)
Associated disorders	Multiple sclerosis, ADEM, leukodystrophies	Guillain-Barré, CMT, CIDP, NF1

Myelination Process

1. Recognition: Axonal signals (neuregulin-1 type III) trigger glial differentiation
2. Ensheathment: Glial process wraps axon spirally
3. Compaction: Cytoplasm expelled; major dense line (cytoplasmic faces) and intraperiod line (extracellular faces) form
4. Maturation: Myelin proteins (MBP, PLP in CNS; P0, PMP22 in PNS) stabilize structure
5. Node of Ranvier formation: Specialized domains (Na⁺ channels, Caspr, contactin) for saltatory conduction

Functional Significance

• Saltatory conduction: Action potential jumps between nodes → 50–100× faster than unmyelinated
• Energy efficiency: Less Na⁺/K⁺-ATPase activity per impulse
• Axonal support: Metabolic coupling, structural integrity

Clinical Correlates

• Demyelination: Conduction block, slowing, ectopic impulses → neurological deficits
• Dysmyelination: Abnormal formation (genetic leukodystrophies)
• Remyelination: Thin, shorter internodes; incomplete functional recovery

Myelin is a glial specialization enabling rapid, efficient neural communication—fundamental to vertebrate nervous system complexity.