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occur, but the damage is not perceived as pain, as the tissue damage does not activate nociceptors, and thus does not cause pain or any protective behavioural changes. This is a well-known phenomenon in internal organs such as the liver or the brain, where a malignant tumour may cause extensive damage that goes unnoticed by the patient (Treede, 2008). The meninges of the brain are nevertheless sensitive and a brain swelling caused by the tumour may cause pain due to activation of nerve endings associated with the meninges. The perception of pain is based on an interaction of receptors, nerves, the spinal cord and the brain including the thalamus and the cortex (Brooks and Tracey, 2005; Treede et al., 2000). Pain receptors are located in skin, muscles, joints, periosteum, most internal organs and around blood vessels. Pain can lead to different experiences (e.g. sharp, dull) as different anatomical structures are involved, and different tissues are characterized by different sensors, density of sensors and different types of fibres. Sharp pain is signaled by A-fibers (conduction time 5-30 m/s) and the reaction time for perception of sharp pain is short. C-fibers (conduction time 0,5-2m/s) are associated with a slower burning type of pain. Both types of nociceptive fibres innervate the skin and deep somatic or visceral structures (Ringkamp and Meyer, 2008; Hellyer et al., 2007). During the slaughter process itself pain can be caused by inappropriate restraint, during incorrectly performed stuns and by tissue damage during the neck cut. There are different types of pain, of which two are welfare relevant during the short time frame of the slaughter process. Phasic or nociceptive pain results from mechanical or thermal stimuli is also called “brief” or “first pain”. Tonic or inflammatory type of pain resulting from chemical stimuli released by injury and inflammations is also called “persistent” or “second pain”. During slaughter both forms of pain are produced. Nociceptive pain is produced by mechanical forces of cutting and inflammatory pain immediately thereafter by tissue damage. The severity of inflammatory pain can be reduced but not eliminated by a clean cut performed with a sharp knife, while this has little or no influence on nociceptive pain (Brooks and Tracey, 2005; Woolf, 2004). The threshold of nociceptors is not constant. Substances from damaged cells or inflamed tissues directly stimulate nociceptors and are considered “nociceptive activators” (e.g. potassium ions or ATP or certain inflammatory mediators). These substances contribute to primary hyperalgesia. A so called “sensitizing soup” sensitizes the nociceptors to subsequent painful and also nonpainful stimuli (Muir, 2007; Hellyer et al., 2007). Pain can be modulated by the central nervous system in both directions (Tracey and Mantyh, 2007). Not all traumata are directly painful, as stress can inhibit the transmission of pain stimuli in brain and spinal cord (Gregory, 2004). This phenomenon called stress-induced analgesia is part of the bodies self protection measures during life-threatening situations, it involves endogenous opioides, which block pain neurotransmission (Zimmermann, 2005). It must be considered in this context that stress induced analgesia does not apply in every life threatening situation and for every individual. Often this involves the individual being involved in very vigorous activity and heightened awareness, frequently associated with emergency physiological responses. This can apply to fighting or other dangerous and demanding activities (Bodnar, 1984). The possibility exists that animals which are to be slaughtered might be in such a state but with correct pre-slaughter handling this would not be the routine situation. Furthermore, only around 30-40% of humans experience stress induced analgesia in an emergency situation (Melzack et al., 1982). Hence it is likely that endogenousopioid-induced analgesia may not often occur during slaughter. This can be underlined by practitioners reports of animal pain reactions during stressful situations. Cattle for example, being restrained for claw trimming and showing obvious stress symptoms (wide open eyes, vocalisation) still react immensely when e.g. the bandage is taken off an inflamed claw.