Which of the following accurately describes warmth detection by receptors?

• A. Warmth is detected by temperature-sensitive free nerve endings in hair follicles
• B. Warmth receptors detect temperatures above about 30°C, increasing firing to ~46°C then pain
• C. Warmth is detected only by Merkel cells
• D. Warmth is detected by Pacinian corpuscles in the deep dermis

Answer: (B)

Warmth detection comes from temperature-sensitive free nerve endings in the skin. These warm receptors begin firing when skin temperature rises above about 30°C, and their firing rate increases as the temperature climbs toward the mid-40s; around 46°C, the response tends to elicit pain to protect tissue from damage. These receptors are free nerve endings in the dermis, not located in hair follicles, and they are distinct from Merkel cells (which are for touch and texture) and Pacinian corpuscles (which detect deep pressure and vibration). So, warm sensation is produced by these temperature-sensitive free nerve endings that progressively increase activity with warming and trigger pain at potentially damaging temperatures.