On electron microscopy, a nerve fiber shows a thick myelin sheath with visible Schmidt-Lanterman clefts and a single Schwann cell wrapping one internodal segment. This appearance indicates which type of nerve fiber?

• A Myelinated central nervous system fiber myelinated by oligodendrocytes
• B Unmyelinated C fiber wrapped by a single Schwann cell
• C Myelinated peripheral nerve fiber (A-fiber or B-fiber in the peripheral nervous system) ✓
• D Axon with compact myelin and no Schmidt-Lanterman clefts, indicating CNS myelin

Explanation

Schmidt-Lanterman clefts are funnel-shaped cytoplasmic channels within the compact myelin of peripheral nerve fibers, representing zones where the Schwann cell cytoplasm has not been entirely extruded during myelination. They are characteristic of peripheral myelin (PNS, formed by Schwann cells) and are not found in CNS myelin (formed by oligodendrocytes, which produce compact myelin without these clefts). Each Schwann cell myelinates one internodal segment of one axon in the PNS. Unmyelinated C-fibers are embedded in Schwann cell cytoplasm (Remak bundles) without a myelin sheath.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.