Chapter 1 Pain and hyperalgesia: definitions and theories

Abstract

This chapter describes pain as an unpleasant sensory and emotional experience associated with actual or potential tissue damage. It reflects the first-person perspective of pain: “pain is an unpleasant sensory and emotional experience.” “Nociception” is a term that may more adequately reflect these aspects of pain sensation. The chapter reviews that the adequate stimulus to activate the receptive organs of the nociceptive system consists of either actual or potential tissue damage. But not all noxious stimuli are detected by the nociceptive system. Therefore, the adequate stimulus for this system in the strict sense is that subset of noxious stimuli that can be encoded by the nociceptive system. It is not unusual for a sensory system to encode only a part of the range of environmental phenomena that its receptive organs are specialized for: visual stimuli consist of a restricted range of wavelengths of electromagnetic waves, and auditory stimuli consist of a restricted frequency range of pressure waves in the air. The positive sensory sign of nociception is called “hyperalgesia”. It is clinically important, because it is a source of suffering that may become independent of a warning function. Pain because of activation of non-nociceptive afferents that then excite sensitized second-order neurons is a special case of hyperalgesia with its own term “allodynia”.