FUNDAMENTALS OF PAIN PERCEPTION IN ANIMALS

Pain is mediated by functionally distinctive components. It may involv e acute high threshold afferent stimuli (thermal, mechanical or chemic ally damaged tissue), protracted afferent input (long-lasting hyperalg esia) or low-threshold input (allodynia as related to pain from light touch). Behavioral patterns will be associated with the effects of nox ious stimulus on excitatory transmitters. Humane studies using mechani cal, thermal (cold presser test), audio-evoked potentials, or other no xious stimuli during anesthesia and analgesia provide clues to percept ion of pain in animals and help us determine guidelines for the clinic al relief of animal pain. There is a better understanding of cutaneous somatosensory responses than for deep sensation (e.g., subcutaneous t issue, muscle, bone, visceral. The prevention or treatment of pain can best be accomplished when there is a diagnosis of the neuroanatomical distribution of the pain based on evidence of sensory dysfunction inv olving a peripheral nerve, plexus, nerve root or central pathway using different modalities for quantitative sensory testing. Anatomical stu dies have demonstrated that unmyelinated primary afferent fibers conta in a variety of neuroactive substances which may be released by high-i ntensity peripheral stimulation. Fast depolarization of the dorsal hor n nociresponsive neurons is mediated by tissue damage. These neurorece ptors may be activated by glutamate, substance P, and neurokinin A. Th e major ascending pathway relaying nociceptive information relating to pain is the spinothalamic tract. Most of the neurons in this tract ar e nociceptive or thermoreceptive. The thalamus is the final relay nucl eus for all the sensory systems, except olfactory tissue. Injury or di sease processes in deeper tissue or visceral areas with extensive inne rvation may result in pain thresholds not adequately controlled by cur rent available analgesics or at their recommended dosage levels. These issues are complex and diagnosis is even more difficult due to specie s and breed differences in outward expression to painful insult. Medic ations can now be targeted for specific receptors. (C) 1998 Elsevier S cience B.V. All rights reserved.