Stimulation of polymodal C-fiber receptors in peripheral nerve endings causes transmission of sensation of pain.
They are stimulated by chemical, thermal, and mechanical stimuli; the thermal stimuli may include warmth or cold.

Suzuki R, Sikandar S, Dickenson AH. Nociception: basic principles. In: Bruera ED, Portenoy RK (eds) Cancer Pain Assessment and Management. Cambridge University Press. Cambridge. 2010. p. 4.