The counterirritant effect produced by infrared radiation is best explained by

• A. The increase in venous and arterial capillary pressure
• B. A desensitization of superficial sensory nerves
• C. The decrease of stimuli passing over sensory nerves
• D. The sedative effect produced by mild heating

Answer: (B)

Infrared heating creates a counterirritant effect by desensitizing superficial sensory nerves, which aligns with the way non-noxious cutaneous input can modulate pain signals. The warmth from infrared stimulates skin receptors and, through mechanisms like the gate-control idea, reduces the transmission of deeper nociceptive signals to the brain. In other words, the superficial nerves become less responsive, so pain signals are dampened. This is why desensitization of these superficial nerves best explains the counterirritant effect. The other options don’t describe this specific neural modulation: vascular changes and general reductions in stimuli don’t capture the targeted nerve desensitization that leads to analgesia, and a sedative effect from mild heating isn’t the mechanism at play here.