Nerve endings which detect and respond to painful or unpleasant stimuli.
In animals, nerve endings that signal the sensation of pain. In humans, they are called pain receptors.
Nerve endings that signal the sensation of pain. 
Peripheral receptors for pain. Nociceptors include receptors which are sensitive to painful mechanical stimuli, extreme heat or cold, and chemical stimuli. All nociceptors are free nerve endings. ... (12 Dec 1998) ... 
Nerve endings which detect and respond to painful or unpleasant stimuli.
Peripheral receptors for pain which sense painful stimuli and transmit signals to the central nervous system where the sensation of pain is perceived.