A-Delta Fibres Pain — FRCA Primary MCQ
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Correct answer: D — Lightly myelinated; 5–30 m/s; sharp, well-localised first pain
A-delta fibres are small-diameter, lightly myelinated primary afferents. Myelination permits conduction at approximately 5–30 m/s, producing the rapid, sharp and relatively well-localised sensation termed first pain. This is faster than transmission by unmyelinated C fibres, which conduct at about 0.5–2 m/s and produce delayed, dull or burning second pain (option B). Option C describes large A-alpha proprioceptive afferents, while option A describes large A-beta mechanosensory fibres. Option B describes myelinated preganglionic autonomic B fibres rather than somatic nociceptive afferents. Thus, “fast” is relative to C-fibre pain transmission; A-delta fibres remain slower than the large, heavily myelinated A-alpha and A-beta classes.
Reference: Boezaart AP et al. Visceral versus somatic pain: an educational review of anatomy and clinical implications. Regional Anesthesia & Pain Medicine. 2021;46:629–636. https://rapm.bmj.com/content/46/7/629