Primary afferent fibres responding to high-threshold noxious stimuli (noxious thermal, mechanical, chemical) are called nociceptors. They either belong to Aδ or C fibres. These fibres may have specialised nerve endings like Meissner’s corpuscles and Pacinian corpuscles. Nociceptors have their cell bodies in the dorsal horn. Stimulation of Aδ fibres leads to localised, sharp, pricking pain, whereas C fibres are involved with dull or poorly localised pain.

They develop from neural crest stem cells. They undergo further differentiation to peptidergic and nonpeptidergic nociceptors. The fully formed nociceptor expresses numerous ion channels and receptors. Sensory specificity is established by repeated exposure to a certain specific stimuli.

Their response threshold is usually higher than receptors responding to mechanical or thermal stimuli. They also have the ability to increase the firing frequency of action potentials in response to nociceptive stimuli.

Classification is either based on size or on function.

Classification by size: