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Physiolohy Of Pain
Presented by :
Dr. Monir O. Mhemed
Abusharib
1st year A@E master
student
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Definition
Pain is an unpleasant sensory andemotional experience associated withactual or potential tissue damage
Although acute pain and associatedresponses can be unpleasant and oftendebilitating, they serve important adaptive
purposes. They identify and localize
noxious stimuli, initiate withdrawalresponses that limit tissue injury,
inhibit mobility thereby enhancing woundhealing.
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Classifications
Pain has been classified into two major types
fast pain and slow pain.
Fast pain is felt within about 0.1 second after
a pain is applied, whereas slow pain stimulus
begins only after 1 second or more and thenincreases slowly over many seconds andsometimes even minutes.
the conduction pathways for these two types ofpain are different .
Fast pain is also called sharp pain,acute pain.Slow pain also goes by many names, such asslow burning pain, aching pain, and chronic pain.This type of pain is usually associated with tissuedestruction.
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Pain receptors
Pain Receptors Are Free Nerve Endingswhich are also called nociceptors, theircell body lies in dorsal root ganglion and
the proximal end terminate at posteriorhorn of spinal cord. They are widespreadin the superficial layers of the skin as wellas in certain internal tissues, such as:
theperiosteum, the arterialwalls, thejointsurfaces, and the falxand tentorium in the
cranial vault.
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Most other deep tissues are only
sparsely supplied with pain endings;
nevertheless, any widespread tissuedamage can summate to cause
the slow-chronic-aching type of pain
in most of these areas
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Three Types of Stimuli Excite PainReceptors:
Mechanical, Thermal, and Chemical.
In general, fast pain is elicited by themechanical and thermal types of Stimuli.
whereas slow pain can be elicited by allthree types
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Noxious stimuli are converted into a calcium ion(Ca2+) mediated electrical depolarization withinthe nociceptor
Some of the chemicals that excite the chemical
type of pain are bradykinin, serotonin, histamine,potassium ions, H, acetylcholine, and proteolytic
enzymes.
In addition, prostaglandins and substance Penhance the sensitivity of pain endings but do not
directly excite them.The chemical substances are especially importantin stimulating the slow, suffering type of pain thatoccurs after tissue injury
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In contrast to most other sensory receptors of the body,
pain receptors adapt very little and sometimes not at all.
In fact, under some conditions, excitation of pain fibers
becomes progressively greater, especially so for
slow-aching-nauseous pain, as the pain stimulus
continues.This increase in sensitivity of the pain receptors
is called hyperalgesia. One can readily understand
the importance of this failure of pain receptors to
adapt, because it allows the pain to keep the person
apprised of a tissue-damaging stimulus as long as itpersists.
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second-order neurons cross immediately to theopposite side of the cord through the anteriorcommissure and then turn upward, passing to
the brain in the anterolateralcolumns.
Termination of the Neospinothalamic Tract inthe Brain Stem and Thalamus.A few fibers
of the neospinothalamic tract terminate in thereticular areas of the brain stem, but most pass
all the way to the thalamus without interruption,
terminating in the ventrobasalcomplexalong withthe dorsal column tract for tactile sensations.
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Localization offast pain
Fast pain is well localized ,however
when only pain receptor is stimulatedwithout simultaneous simulation of
tactile receptor even fast pain will be
poorly localized
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continue
Only 1/10- 1/4 of the fiber terminate
in the thalamus,most of the fibers
terminate at:
Reticular nuclei in the medulla,pons,
periaqueductal grey region
surrounding the aqueduct
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3rd order neuron project to the
somatosensory area in the cortex
where it relay on 4th order neurons
Cerebral cortex plays an essential
role in interpreting the quality of pain,
even though pain perception is the
function of the lower centres
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Poor localization of pain is due tomultisynaptic,diffuse connectivity of
this pathway
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Reffered pain
Often a person feels pain in a part of
the body that is fairly remote from the
tissue causing the pain. This is calledreferred pain.
Pain referto astructure that originate
from the same embryonicsegment ofthe structure at which pain originate
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Because both neurons relay on thesame 2nd order neuron
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Descending inhibitory pathway
Pain is modulated at spinal level by
inhibitory tracts originate from:
cerebral cortex, hypothalamus,
thalamus,PAG region, these neurons
stimulates inhibitory intrneurons.
These inhibitory pathway are
influenced by pts psychological andemotional status
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anxiety, psychological stress and
depression reduce the activity of theinhibitory pathway, thereby lowering
the threshold for pain and increasing
pain sensetivity score
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Hyperalgesia
Defined as state of increased pain
sensitivity and enhanced perception
following acute injury that may persist
chronically.
The hyperalgesic region may extend to
dermatomes above and below the area
of injury and is associated with ipsilateral(and occasionally contralateral)
muscular spasm/immobility.
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continue
Primaryhyperalgesia
Increased pain sensitivity at the injurysite Related to peripheral release of
intracellular or humoral noxious
mediators
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Secondaryhyperalgesia
Increased pain sensitivity at adjacent,uninjured sites Related to changes in
excitability of spinal and supraspinal
neurons
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The end
Thank you