During a compound nerve action potential study, a myelinated nerve fiber conducts at 60 m/s. What is the approximate axon diameter of this fiber, and which Erlanger-Gasser classification does it belong to?

• A Diameter ~12 μm; Aβ fiber (6–12 μm → 35–75 m/s), carrying tactile discrimination and proprioception ✓
• B Diameter ~6 μm; Aδ fiber (1–6 μm → 5–40 m/s), carrying sharp pain and cold sensation
• C Diameter ~20 μm; Aα fiber (12–20 μm → 70–120 m/s), carrying motor signals to extrafusal muscle fibers
• D Diameter ~6 μm; B fiber (3 μm → 3–15 m/s), preganglionic autonomic fibers

Explanation

Conduction velocity in myelinated fibers approximates 6 times the diameter in microns (or: velocity in m/s ≈ 6 × diameter in μm). At 60 m/s, diameter ≈ 60/6 = 10 μm, which falls in the Aβ range (6–12 μm, velocity 35–75 m/s). Aβ fibers carry touch, pressure, vibration, and proprioceptive signals from Meissner's corpuscles, Pacinian corpuscles, and Merkel cells. Aα fibers (option C) conduct at 70–120 m/s (diameter 12–20 μm), carrying efferent signals to alpha motor neurons and Ia afferents from muscle spindles. Aδ fibers (option B) are thinner (1–6 μm) and conduct at 5–40 m/s. B fibers (option D) are preganglionic autonomic fibers conducting at 3–15 m/s.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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