PHYSIOLOGY OF PAINPHYSIOLOGY OF PAIN

Department of physiologyDepartment of physiology

Definitions of painDefinitions of pain

the International Association for the Study the International Association for the Study of Pain (IASP) introduced the definition of of Pain (IASP) introduced the definition of pain.pain. It is an “unpleasant sensory and emotionalIt is an “unpleasant sensory and emotional experience associated with actual or experience associated with actual or potential tissue damage, or described in potential tissue damage, or described in terms of such damage”.terms of such damage”.

Some key points:Some key points:

Pain is one of the body’s defensive mechanisms that signalizes about the danger to the organism; The sensation of pain is basic to all people; It is a personal experience that all humans endure; Acute pain is a primary reason why people seek medical help; Pain is a complex phenomenon that involves sensory, behavioural, emotional and cultural components.

The experience of painThe experience of painThree systems interact usually to produce pain:Three systems interact usually to produce pain:1.1. SensorySensory - discriminative- discriminative

2. 2. MotivationalMotivational - affective- affective

3. 3. CognitiveCognitive - evaluative- evaluative

1. 1. Sensory - discriminative systemSensory - discriminative system processes information about processes information about tthehe strength, intensity, strength, intensity, quality quality and temporal and spatial and temporal and spatial aspects of painaspects of pain

2. 2. Motivational - affective systemMotivational - affective system determines the individual´s determines the individual´s approach-avoidance behaviours (depression, anxiety)approach-avoidance behaviours (depression, anxiety)

3. 3. Cognitive - evaluative systemCognitive - evaluative system thoughts concerning the cause thoughts concerning the cause and significance of pain and significance of pain

Classical description of pain Classical description of pain includes includes

4 processes:4 processes:

1.1. TransductionTransduction

2.2. TransmissionTransmission

3.3. PerceptionPerception

4.4. ModulationModulation

TransductionTransduction

It is a process of It is a process of the conversion of the energy the conversion of the energy

from a noxious thermal, mechanical, or from a noxious thermal, mechanical, or

chemical stimulus into electrical energy (nerve chemical stimulus into electrical energy (nerve

impulses) by sensory receptors called impulses) by sensory receptors called

nociceptors.nociceptors.

Receptors for pain are called Receptors for pain are called nociceptorsnociceptors..

Stimulus that causes pain is called Stimulus that causes pain is called noxious noxious

stimulusstimulus..

NociceptiveNociceptive (“pain”) and (“pain”) and anti-nociceptiveanti-nociceptive (“anti- (“anti-

pain”) systems can be distinguished.pain”) systems can be distinguished.

TransductionTransduction

Nociceptors:Nociceptors: EndEndingings of small unmyelinated and lightly s of small unmyelinated and lightly myelinated afferent neuronsmyelinated afferent neurons

Stimulators:Stimulators: CChemical, mechanical and thermal hemical, mechanical and thermal noxnoxious ious

stimulistimuli

Mild stimulationMild stimulation positive, pleasurable positive, pleasurable sensationsensation (e.g. tickling)(e.g. tickling)

Strong stimulationStrong stimulation painpain

ThTheseese differences are a result of the differences are a result of the frequency frequency and amplitudeand amplitude of the afferent signalof the afferent signal transmittedtransmitted from the nerve endingsfrom the nerve endings to the CNS to the CNS

LocationLocation: : IIn muscles, tendons, epidermisn muscles, tendons, epidermis,, subcutanous subcutanous

tissue,tissue, visceral organsvisceral organs

- - they are not evenly distributed in the bodythey are not evenly distributed in the body (in skin more then in internal structures(in skin more then in internal structures))

TransductionTransductionInjury to tissue causes cells to break down and release

various tissue by products and mediators of

inflammation:

prostaglandins

substance P

bradykinin

histamine

serotonin

cytokines

All these cause the activation

of nociceptors!

TransmissionTransmission

Signals from nociceptors are transmitted Signals from nociceptors are transmitted via:via:

A(alfa)- B(beta)-D(delta) fibersA(alfa)- B(beta)-D(delta) fibers (small (small myelinated fibers) myelinated fibers) rapidly conducting rapidly conducting (acute pain);(acute pain);

C-fibersC-fibers (unmyelinated) (unmyelinated) slowly conducting (prolonged “dull slowly conducting (prolonged “dull pain”)pain”)

Afferent pathways from nociceptors Afferent pathways from nociceptors terminate in theterminate in the dorsal horndorsal horn (DH)(DH) of of thethe spinalspinal cord cord from DH neurons the information from DH neurons the information is further transmitted via is further transmitted via spinothalamic tractspinothalamic tract to thalamus to thalamus and than to: and than to: reticular formation of brainstem; reticular formation of brainstem; hypothalamus; hypothalamus; somasonesory cortex; somasonesory cortex; limbic system.. limbic system..

PerceptionPerception

It is It is an uncomfortable awareness of some part of the body, an uncomfortable awareness of some part of the body,

characterized by a distinctly unpleasant sensation and characterized by a distinctly unpleasant sensation and

negative emotion, best described as threat.negative emotion, best described as threat.

Upon perception affective changes take place having:Upon perception affective changes take place having:

• emotional aspectemotional aspect

• behavioral aspectbehavioral aspect

• social (environmental) aspectsocial (environmental) aspect

• memorizing aspectmemorizing aspect

ModulationModulation

It is It is a process of a process of “modification”“modification” of pain. of pain.

Descending pain inhibitory pathways from brain to DH Descending pain inhibitory pathways from brain to DH

( dorsal horn) neurons exist. ( dorsal horn) neurons exist.

Following mediators are released from them:Following mediators are released from them:

• endogenous opioids (endorphins, encephalins)endogenous opioids (endorphins, encephalins)

• Serotonin, noradrenalinSerotonin, noradrenalin

• GABAGABA

All of them inhibit the transmission of pain stimuli in DH

neurons (the anti-nociceptive system).

ModulationModulation

Theory of Theory of ppain productionain production and and modulationmodulation

Most rational Most rational explanation explanation of pain of pain production and modulationproduction and modulation

isis based on based on gate control theorygate control theory (created by Melzack and Wall (created by Melzack and Wall))..

According to this theory neurons of DH function as a gate,According to this theory neurons of DH function as a gate,

regulating transmission of impulses to CNS!regulating transmission of impulses to CNS!

I – inhibitory neuronP – projection neuron

No stimulation at all small/large fibers are quiet I neuron is active / P neuron is not active “gate is closed” no painNon painful stimulation large fibers are active both I neuron and P neurons are active “gate is closed” no painPainful stimulation small fibers are active I neuron is blocked and P neuron is excited “gate is opened” pain reaches the brain

Pain Pain classificationclassification

1.1. Somatogenic painSomatogenic pain is pain with cause (usually known)is pain with cause (usually known) localilocalizzed in the body tissueed in the body tissue

a/ a/ nociceptive painnociceptive pain (activation of nociceptors)(activation of nociceptors)

superficial somatic – superficial somatic – originating from skin and superficial tissuesoriginating from skin and superficial tissues(sharp, well-defined, localized);(sharp, well-defined, localized);

deep somatic – deep somatic – originating from joints, tendons, bones, musclesoriginating from joints, tendons, bones, muscles(dull, aching, poorly-localized with longer duration);(dull, aching, poorly-localized with longer duration);

visceral – visceral – originating from visceral organs (can be different);originating from visceral organs (can be different);

b/ b/ neuropatneuropathhic painic pain – – pain is caused by the damage within pain is caused by the damage within the peripheral or central nervous systemsthe peripheral or central nervous systems

22. . Psychogenic painPsychogenic pain is pain for which there is no known is pain for which there is no known physical cause,physical cause, but processing of sensitive information but processing of sensitive information in CNS is din CNS is diisturbedsturbed

Pain ClassificationPain Classification

According to duration:According to duration:

Acute painAcute pain is a protective mechanism that alerts the is a protective mechanism that alerts the

individual to a condition or experience that is individual to a condition or experience that is

immediately immediately

harmful to the bodyharmful to the body

OnsetOnset -- usually suddenusually suddenReliefRelief -- after removal of the chemical mediatorsafter removal of the chemical mediators from from nociceptorsnociceptors

•• This type of pain mobilises the individual to prompt action This type of pain mobilises the individual to prompt action to relief itto relief it

•• Stimulation of autonomicStimulation of autonomic nervous system can be observed nervous system can be observed during this type of painduring this type of pain (mydriasis, tachycardia, tachypnoe, (mydriasis, tachycardia, tachypnoe,

sweating, vasoconstriction)sweating, vasoconstriction)

Chronic painChronic pain is persistentis persistent or intermittent or intermittent usually defined usually defined

as lasting at least as lasting at least 6 months6 months or more or more

The cause of it is often unknownThe cause of it is often unknown, , vvery ery often it is often it is

associated with a sense of hopelessness and associated with a sense of hopelessness and

helplessness.helplessness. Depression often results.Depression often results.

Pain classification Pain classification

Chronic pain producesChronic pain produces significant significant behavioural and behavioural and psychological changespsychological changes

The main changes are:The main changes are:

- - depression depression

- sleeping disorders- sleeping disorders

- - preoccupation with the pain preoccupation with the pain

- tendency to deny pain- tendency to deny pain

Pain threshold and Pain threshold and ppain ain ttoleranceolerance

The pain thresholdThe pain threshold is the point at which a stimulus is perceivedis the point at which a stimulus is perceivedas pain.as pain.It does not vary significantly among It does not vary significantly among healthy healthy people or in the same people or in the same person over time.person over time.

The pain tolerance is expressed as duration of time or the

intensity of pain that an individual will endure before

initiation

overt pain responses.

It is influenced by - persons cultural prescriptions - expectations - role behaviours - physical and mental health

Pain tolerancePain tolerance is generallyis generally decreaseddecreased::

-- with repeated exposure to pain with repeated exposure to pain

- - by fatigue, anger, apprehensionby fatigue, anger, apprehension

- - sleep deprivationsleep deprivation

Tolerance to painTolerance to pain may bemay be increasedincreased::

- - by alcohol consumption by alcohol consumption

- - medication, hypnosismedication, hypnosis

- - warmth, distracting activities warmth, distracting activities

- - strong beliefs or faithstrong beliefs or faith

Pain tolerancePain tolerance varies greatlyvaries greatly among people and in among people and in the samethe same person over timeperson over time

A decrease in pain tolerance is also evident in the elderly, A decrease in pain tolerance is also evident in the elderly,

and women appear to be more and women appear to be more tolerant tolerant to pain than to pain than

menmen

Pain characteristics that should be Pain characteristics that should be found out in order to establish found out in order to establish correct diagnosis:correct diagnosis:

QualityQuality IntensityIntensity LocalizationLocalization RadiationRadiation Frequency and durationFrequency and duration Onset and offsetOnset and offset Exacerbating factorsExacerbating factors Relief factorsRelief factors

Some disorders of pain perception include:

hyperalgesia – increased response to a stimulus which is normally painful;

hypoalgesia – decreased response to a stimulus which is normally painful;

analgesia – no response to painful stimulus;

allodyniaallodynia – phenomenon characterised by – phenomenon characterised by painful sensations provoked by nonpainful sensations provoked by non--noxious noxious stimuli, stimuli, ((e.g. touche.g. touch))

Physiological basis of pain relief Physiological basis of pain relief

Methods of pain relief : Methods of pain relief : 1) 1) Physical methodsPhysical methods- temperature, - temperature, tactile stimulation, electrical .... tactile stimulation, electrical .... 2) 2) Pharmacological methodsPharmacological methods- - use of drugs analgesic action. use of drugs analgesic action. a)a) local anesthesia, local anesthesia, b)b) conduction anesthesia, conduction anesthesia, c)c)central central anesthesia(narcosis). 3) anesthesia(narcosis). 3) Surgical Surgical methodsmethods-section of the nerve fibers or -section of the nerve fibers or destruction centers of pain. 4) destruction centers of pain. 4) Psychotherapeutic methodsPsychotherapeutic methods-principles of -principles of meditation, yoga, hypnosis .meditation, yoga, hypnosis .

Give..

..Hope!