Pain perception

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.

Transmission to the CNS by nociceptive fibres, which terminate primarily in laminae I (marginal zone) and II (substantia gelatinosa) of the superficial dorsal horn of the spinal cord.

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;

Neuromodulation;

Neurotransmitters.

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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