We have several types of heat sensors in our nervous system. They are triggered by heat energy, not by radiation, so it doesn’t matter what frequency incoming electromagnetic radiation is responsible for the heat, or if the heat enters the body through conduction or convection.
Heat, cold, and pain are all sensed through what are called Transient receptor potential (TRP) channels. Basically, the channels respond to heat and allow ions to flow into neurons, which our nervous system interprets as a heat sense. There are several different types of TRP channels in humans. The ones responsible for heat are called TRPA1, TRPM8, TRPV1, TRPV2, TRPV3, and TRPV4. These cover a wide range of temperatures. In order of coldest to warmest, they go TRPA1, TRPM8, TRPV4, TRPV3, TRPV1, TRPV2.
The last two, TRPV1 and TRPV2, trigger a pain response. TRPV1 kicks on at about 45 deg C, where TRPV2 kicks on at about 50 deg C (just below where tissue damage actually starts to occur). TRPV1 also reacts to capsaicin, which is why hot peppers feel “hot” to us. Many animals lack the TRPV1 sensor and can eat hot peppers without feeling the “hot”.
TRPA1 also triggers a cold pain response.