The primary mechanism by which superficial heat relieves pain is believed to involve

• A. Endorphin release
• B. Decreased nerve conduction velocity
• C. Gate Control Theory
• D. Activation of A-delta fibers

Answer: (C)

Warmth relieves pain mainly through the Gate Control Theory. When the skin is warmed, thermoreceptors are activated and signals travel through large-diameter A-beta fibers to the spinal cord. These non-painful inputs activate inhibitory interneurons in the dorsal horn, dampening the transmission of pain signals carried by nociceptive fibers (C fibers and A-delta fibers) to higher brain centers. In this way, the “gate” at the spinal level is closed to some extent, reducing the amount of pain information that reaches the brain.

Endorphin release can contribute to analgesia in some contexts, but the immediate effect of superficial heating is best explained by spinal gating rather than opioid-mediated mechanisms. Decreased nerve conduction velocity is not a typical result of heating (it’s more associated with cooling), and activation of A-delta fibers would tend to convey pain rather than relieve it.