Which feature best distinguishes fast (A-delta) pain fibers from slow (C) pain fibers in the spinothalamic tract?

Explanation

## Distinguishing Features of A-delta vs C Pain Fibers ### Structural and Functional Comparison | Feature | A-delta Fibers | C Fibers | |---------|---|---| | **Myelination** | Myelinated | Unmyelinated | | **Conduction Velocity** | 5–30 m/s | 0.5–2 m/s | | **Fiber Diameter** | 2–5 μm | 0.4–1.2 μm | | **Pain Quality** | Sharp, localized, first pain | Burning, diffuse, second pain | | **Receptor Type** | Mechanical nociceptors (high threshold) | Polymodal nociceptors (thermal, chemical, mechanical) | | **Dorsal Horn Laminae** | Laminae I & V | Laminae I & II (substantia gelatinosa) | | **Neurotransmitter** | Glutamate + Substance P | Glutamate + Substance P + CGRP | **Key Point:** The MOST DISCRIMINATING feature is **myelination and conduction velocity**. A-delta fibers are myelinated and conduct rapidly (5–30 m/s), producing the sharp, immediate pain sensation. C fibers are unmyelinated and conduct slowly (0.5–2 m/s), responsible for the delayed burning pain. ### Clinical Correlation **Clinical Pearl:** When you touch a hot surface, you first feel sharp pain (A-delta → fast conduction) followed seconds later by burning pain (C → slow conduction). This dual sensation is the hallmark of their functional difference. **High-Yield:** Spinal cord injury affecting the spinothalamic tract produces loss of pain and temperature sensation *below* the lesion, with A-delta and C fibers both decussating at the spinal level before ascending. ### Why Other Options Are Incomplete - **Neurotransmitter:** Both release glutamate and Substance P; C fibers additionally release CGRP, but this is not the PRIMARY discriminator. - **Receptive field & adaptation:** Related to function but less fundamental than the structural difference in myelination. - **Laminae of termination:** A-delta terminate in laminae I & V; C in laminae I & II — this is a consequence of their different receptor types, not the primary discriminator. [cite:Guyton & Hall Textbook of Medical Physiology 14e Ch 48]