How mammals feel and express pain?

According to the IASP, an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage . In other words, more than what animal perceive, pain correspond to general feeling and past history. All vertebrates possess primary nervous structures that take part in the treatment of nociceptive information, which mean directly linked to the perception of pain (spinal cord, thalamus, limbic system). In mammals, the development of the cerebral cortex leads to supplementary cognitive and emotional factors that would modulate the perception. Origins of pain are diverse: irritation or injury, nervous diseases or even psychogenic disorders. Free nerves fibres, widespread in organs and tissues, would convey the nociceptive message as soon as a generating pain stimuli or substance would touch the body. The impulse conveyed to the spinal cord will lead to 1) automatic motor response (reflex) and via the integration of emotional (amygdala) and cognitive (cortex) factors to 2) the elaboration of adaptative strategies that would lessen the damages and the pain of the moment but also decrease the risk of a new painful event. The ways mammals express pain vary a lot according to the species (prey vs. predator) but also between subjects (linked to the personality). Besides the evaluation physiological parameters which require invasive methods or a close proximity, the observation of the behaviours display represents a good way to assess the general state of the animal. Environmental factors, physical but also social, would modify the expressions of these behaviours related to the expression of pain or suffering. Besides the specific signs of pain (not numerous, often similar to those express in stress or fearful situations), normal behaviour is the main thing which is mainly modified. It is compulsory to have a great understanding and knowledge of the normal behavioural repertoire of the different species in order to detect at a very early point subtle modifications that can suggest any suffering. Some examples of signs related to the expression of pain in rodents, rabbits, dogs and cats and also farm animals will be presented.