Recognition and Alleviation of Pain and Distress in Laboratory Animals (1992) / Chapter Skim
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2 The Basis of Pain
2 The Basis of Pain
Pages 10-16
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From page 10... ...
An alternative theory proposed that specialized receptors in peripheral tissues encoded features of tissue-damaging stimuli-their quality, location, intensity, and duration. The theory of neuronal specialization in the pain sensing system has received considerable support from the finding of specialized receptors that signal tissue damage (Dubner and Bennett, 10
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From page 11... ...
Arachidonic acid is released after cell injury from phospholipids embedded in cell membranes. Metabolism proceeds in two directions: The enzyme cyclo-oxygenase converts arachidonic acid to prostaglandins, which increase vascular permeability, activate leukocyte migration, and sensitize nociceptors; and the enzyme lipoxygenase results in the formation of leukotrienes, some of which increase vascular permeability end chemotaxis of polymorphonuclearleukocytes.
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From page 12... ...
Four major long projection pathways appear to be important in nociceptive transmission (Figure 2-11: the spinothalamic tract, the spinocervical tract, the spinomesencephalic tract, and the dorsal column postsynaptic spinomedullary system (Dubner and Bennett, 1983~. The relative importance of each system is not entirely clear and likely depends on the species.
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From page 13... ...
Spinocervical tract neurons are mainly low-threshold mechanoreceptive neurons, responding best to light touch or hair stimulation; a few are nociceptive neurons almost entirely of the wide-dynamic-range type. The spinomesencephalic tract is known to be present in rats, cats, and monkeys and sends bilateral projections
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From page 14... ...
THALAMOCORTICAL MECHANISMS The presence of terminations of the spinothalamic track in the ventrobasal and medial thalamus leaves little doubt that the thalamus plays an important role in pain (Willis, 1985~. There is considerable evidence that neurons in the ventrobasal thalamus respond to tissue-damaging stimuli and have characteristics similar to those of the wide-dynamic-range and nociceptive-specific neurons in the spinal dorsal horn.
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From page 15... ...
Earlier findings in humans that lesions of the postcentral gyrus reduce pain and that stimulation of the exposed somatosensory cerebral cortex can sometimes produce pain constitute additional evidence of a role of the cerebral cortex in the elaboration of pain sensations. DESCENDING CONTROL MECHANISMS The above findings support the involvement of specialized neural pathways in the encoding of pain sensations.
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From page 16... ...
In addition to the opioid peptides, which probably contribute to analgesic processes in the CNS in a more localized fashion via relatively short neural circuits, longer, chemically specific descending neural systems containing norepinephrine or serotonin play major roles in the modulation of pain (Figure 2-2~. Descending norepinephrine- or serotonincontaining pathways, originating in the medulla and pons, affect the output of widedynamic-range and nociceptive-specific neurons and alter responsiveness to nox .
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