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Pain anatomy, theories, and neuromodulation

sensory/discriminative system

processes info about strength, intensity, and time/space aspects of pain, results in prompt withdrawal from painful stimulus; mediated by afferent nerve fibers, spinal cord, brain stem, and higher brain centers

motivational/affective system

determines individual's conditioned or learned approach/avoidance behaviors; mediated by interaction of reticular formation, limbic system, adn brain stem

cognitive/evaluative system

overlies individual's learned behavior concerning experience of pain; may block, modulate, or enhance perception of pain

3 systems that interact to produce pain

sensory/discriminative, motivational/affective , cognitive/evaluative

nociceptors

free nerve endings that respond to chemical, mechanical, and thermal stimuli

nociception

perception of pain

portions of nervous system responsible for sensation and perception of pain

afferent pathways, interpretive centers, efferent pathways

location afferent pathways

PNS, travel to spinal gate in dorsal horn, then ascend to higher centers in CNS

location interpretive centers

brainstem, midbrain, diencephalon, cerebral cortex

efferent pathways

descend from CNS back to dorsal horn of the spinal cord

A(delta) fibers

type of nociceptor, lightly myelinated afferent neurons, carry sharp, well-localized pain sensations, rapid transmission

C fibers

type of nociceptor, small unmyelinated , transmit burning or aching sensations, slow transmission

lateral neospinothalamic tract

carries acute pain info to lateral thalamus, then projets to somatosensory cortex which provides specific pain info

medial paleospinothalamic tract

carries dull and burning pain info to midbrain, is responsible for reflex responses to pain with changes in autonomic function, respiration, circulation, and endocrine function

ventromedial medulla

afferent stimulation of this portion of midbrain stimulates efferent pathways, which modulate or inhibit afferent pain signals at the dorsal horn

periaqueductal gray (PAG)

gray matter surrounding the cerebral aqueduct

periaqueductal gray (PAG)

afferent stimulation of this portion of midbrain stimulates efferent pathways, which modulate or inhibit afferent pain signals at the dorsal horn

specificity theory

pain intensity proportinoal to amount of tissue injury

gate control theory

stimulationof non-nociceptive A fibers (touch, vibration, thermal) causes cells in substantia gelatinosa to close itself off from pain impulses, which diminishes pain sensation

substantia gelatinosa

laminae in the dorsal horn of spinal cord; A and C fibers terminate here

neuromatrix theory of pain

when there is no discernable cause for chronic pain, the brain produces pattern of nerve impulses from widely distributed neural network; impulses can originate independently in brain w/o external input

endorphins

family of neuropeptides that inhibit transmissino of pain impuses in spinal cord and brain

endorphins bind to

mu, kappa, or delta opioid receptors--opiate receptors on the plasma membrane of the afferent neuron; inhibits release of excitatory neurotransmitters

Beta-endorphin

potent endorphin released from hypohalamus and pituitary gland; may result in general sensations of well-being

enkephalin

endorphin found in neurons of brain and spinal cord, weaker analgesic than other endorphins, more potent and longer lasting than morphine

dynorphin

endorphin found in hypothalamus, periaqueductal gray, and spinal dorsal horn; generally impedes pain signals but can incite pain

endomorphins

peptides formed in the brain, highly antinoceceptive, high affinity for morphine receptors. Cause vasodilation by releas of NO from endothelial cells

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