In the histological classification of nerve fibers, which characteristic determines whether a peripheral nerve axon is classified as myelinated or unmyelinated?

• A The diameter of the axon alone
• B The presence of nodes of Ranvier
• C The type of neurotransmitter released at the terminal
• D Whether a Schwann cell forms a myelin sheath (1:1 relationship) or engulfs multiple axons in a mesaxon without forming concentric myelin (1:many relationship) ✓

Explanation

Myelinated nerve fibers have a 1:1 relationship between Schwann cells and axons — each Schwann cell wraps its plasma membrane concentrically around a single axon forming the multilamellar myelin sheath, with periodic nodes of Ranvier (gaps between adjacent Schwann cells). Unmyelinated fibers have a 1:many relationship — one Schwann cell engulfs multiple axons in individual mesaxon troughs without forming concentric wrappings (Remak fibers). Diameter alone does not determine myelination (though myelinated fibers tend to be larger). Saltatory conduction occurs only in myelinated fibers.

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

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