The perception of and responses to pain comprise the following steps:
Transduction: detection of noxious chemical, mechanical and thermal stimuli by nociceptive receptors, with conversion to a neural response. Nociceptors are specialised nerve endings that respond to high threshold noxious stimuli and generally serve a protective function.
Modulation by descending inhibitory responses from the periaqueductal grey matter, nucleus raphe, and reticular formation of the brain stem. These inhibitory pathways descend in the dorso-lateral funiculus to the dorsal horn. The gate control represents control of pain perception by the levels of activity in both nociceptive and non-nociceptive afferent fibres. Anti-nociception is mediated by adrenergic, serotoninergic, GABAergic and opioid mechanisms.
Perception as pain.
Pain modulation is a composite of:
Ascending excitatory afferent pain pathways;
Descending inhibitory pain pathways;
Nociceptor responses may be altered in pathological pain states by the following mechanisms: Reduced threshold for activation; Increased receptive field;
Response to normally non-noxious stimuli (allodynia); Increased intensity of response to noxious stimuli (hyperalgesia); Prolonged post-stimulus sensation: hyperpathia; Emergence of spontaneous activity.
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