Neural basis of nociception

Pain is “an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage”. The processing of sensory signals evoked by noxious stimuli that injure or threaten to destroy tissue is called “nociception”. The weight of evidence indicated that nociception depends on its own sensory receptors, i.e., nociceptors. They are supplied by both small myelinated Aδ fibers and unmyelinated C fibers. There are four different categories of nociceptors, i.e., mechanical, thermal, mechano-thermal and mechanothermochemical (polymodal) nociceptors. Endogenous algogenic substances such as bradykinin activate only polymodal nociceptive fibers. Prostaglandins E2 and I2 sensitize nociceptors.