Animal Pain

Pain is a complex physiological phenomenon; it is hard to define satisfactorily in human beings, and it is extremely difficult to recognize and interpret in animals. Scientific knowledge concerning pain per- ception in animals must be obtained by drawing analogies based on comparative anatomy, physiology, and pathology and by inference based on subjective responses to pain experienced by humans. Debate continues about whether animals of different species perceive pain similarly and whether any species perceives pain the same way hu- mans do. The use of animals in research, in education, and in testing products to minimize adverse effects requires more knowledge about pain perception in animals. Increasing public concern about animal welfare has added urgency to this need. Our knowledge of the scientific basis of the mechanisms of pain has advanced substantially in the last two decades. Nociceptors, or pain receptors, are widespread in the skin and tissues of animals; chemical mediation of nociceptor excitation may provide a key to understanding the peripheral phenomena related to pain. The expression of pain in animals involves multiple ascending and descending branches as well as specialized pain-signaling mechanisms in the spinal cord. The importance of these different pathways varies with species and circum- stances. Endogenous neural systems in the brain stem and forebrain, including both opioid and nonopioid mechanisms, may modulate the central transmission of nociceptive signals in animals.