Which fibre type’s pain is not blocked by opioids?

• A. A-beta fibers.
• B. C fibers.
• C. A-gamma fibers.
• D. A-delta fibers.

Answer: (D)

Opioids work by activating mu receptors in the spinal cord to inhibit the release of excitatory neurotransmitters from nociceptive nerve endings, with a stronger effect on the slow, dull pain carried by C fibers. Fast, sharp pain, which is transmitted by the myelinated A-delta fibers, relies on rapid signal transmission and tends to be less susceptible to this opioid-mediated inhibition. Because of this, pain from A-delta fibers is not blocked as effectively by opioids as pain from C fibers. A-beta fibers carry non-nociceptive touch signals, which are not the target of opioid analgesia for nociceptive pain. So the fast, sharp pain associated with A-delta fibers is the type that opioids block less effectively.