Introduction

Pain varies in intensity, quality, and duration, as well as in its pathophysiologic mechanisms and implications. It is therefore a challenge to define the concept of pain succinctly and accurately. The International Association for the Study of Pain (IASP) defines pain as an unpleasant sensory and emotional experience associated with tissue damage or injury [1], which has been widely and long accepted by the health care professionals. This definition suggests that pain has two important characteristics. One is that pain is an awareness that occurs in the brain and an experience that induced by the activity of multiple cortical sites. Nociception, on the other hand, refers to peripheral or central nervous system activities triggered by noxious stimuli. However, not all nociception brings the sensation of pain. Thus, pain requires the individual’s autonomous consciousness, the participation of intact nervous system as well as the activity of the cortical nociceptive circuitry of the nervous system affecting specific cortical currents. The other is that pain has both sensory and emotional components which are supported by brain imaging studies of patients with specific brain injuries. Imaging results demonstrated that nociception can also lead to activation of corresponding cortical areas that process emotions, such as the amygdala and anterior cingulate cortex which can be controlled by the perceived experiment and further produces different sensations. In addition, patients who suffer from the above-mentioned cortical damage alone may also go through a loss of corresponding sensation, but not lead to the disappearance of pain. They may still feel a vague unpleasant experience when the corresponding brain is injured and the contralateral body part receives noxious stimuli.