md surg pain 2013
TRANSCRIPT
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Physiology of pain
Prof. Vajira Weerasinghe
Professor of Physiology, Faculty of Medicine
University of Peradeniya & Consultant Neurophysiologist, Teaching Hospital, Peradeniya www.slideshare.net/vajira54
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Topics covered in the lecture1. What is pain (International definition of pain)
2. Dual nature of pain: fast pain and slow pain
3. What causes pain : pain stimuli
4. Nerve pathways carrying pain signals to the brain
5. Brain areas involved in pain perception
6. Pain modulatory pathways
7. Neurochemicals involved in pain pathways
8. Gate control theory of pain
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What is pain?• Pain is a difficult word to define
• Patients use different words to describe pain
• eg.• Aching, Pins and needles, Annoying, Pricking, Biting, Hurting,
Radiating, Blunt, Intermittent, Burning, Sore, Miserable, Splitting, Cutting, Nagging, Stabbing, Crawling, Stinging, Crushing, Tender, Dragging, Numbness, Throbbing, Dull, Overwhelming, Tingling, Electric-shock like, Penetrating, Tiring, Excruciating, Piercing, Unbearable
• Different words in Sinhala or in Tamil
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What is pain?• There is an International definition of pain
formulated by the IASP (International Association for the study of pain)
• Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage
IASP – International Association for the Study of Pain 2011
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definition of pain
• It is a symptom• Associated signs are crying, sweating, increased
heart rate, blood pressure, behavioural changes• It is difficult to describe pain although we know what
it is• It is difficult to measure pain
– visual analogue scale is used
• It is a complex sensory modality essential for survival
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Different situations •No stimuli, but pain is felt
•phantom limb pain•eg. in amputated limb
•Stimuli present, but no pain felt•eg. soldier in battle field, sportsman in arena
•Pain due to a stimulus that does not normally provoke pain
•Allodynia
•Pain caused by a lesion or disease of the somatosensory nervous system
•Neuropathic pain
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Pain terminologyInternational Association for the Study of Pain 2011
• Paresthesia– An abnormal sensation, whether spontaneous or evoked
• Dysesthesia– An unpleasant abnormal sensation, whether spontaneous or evoked
• Hyperalgesia – Increased pain from a stimulus that normally provokes pain
• Allodynia – Pain due to a stimulus that does not normally provoke pain
• Hyperesthesia– Increased sensitivity to stimulation, excluding the special senses (increased cutaneous
sensibility to thermal sensation without pain )• Hyperalgesia
– Increased pain from a stimulus that normally provokes pain• Causalgia
– A syndrome of sustained burning pain, allodynia, and hyperpathia after a traumatic nerve lesion, often combined with vasomotor and sudomotor dysfunction and later trophic changes
• Hyperpathia– a painful syndrome characterized by an abnormally painful reaction to a stimulus, especially
a repetitive stimulus, as well as an increased threshold• Neuralgia
– Pain in the distribution of a nerve or nerves
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Pain terminologyInternational Association for the Study of Pain 2011
• Neuropathic Pain – Pain caused by a lesion or disease of the somatosensory nervous system
• Nociceptive pain– Pain that arises from actual or threatened damage to non-neural tissue and is
due to the activation of nociceptors
• Neuropathy– A disturbance of function or pathological change in a nerve: in one nerve,
mononeuropathy; in several nerves, mononeuropathy multiplex; if diffuse and bilateral, polyneuropathy
• Nociception – The neural process of encoding noxious stimuli
• Noxious stimulus– A stimulus that is damaging or threatens damage to normal tissues.
• Pain threshold– The minimum intensity of a stimulus that is perceived as painful.
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• Transduction– Process of converting noxious stimulus to action
potentials
• Perception– Central processing of nociceptive impulses in order
to interpret pain
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Dual nature of painfast and slow pain
• fast pain
– acute– pricking type– well localised– short duration– A fibres are involved– fast conduction 20 m/s– somatic pain
• slow pain
– chronic– throbbing type– poorly localised– long duration– unmyelinated C fibres are
involved– slow conduction 1-2 m/s– visceral pain
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Stimuli • Physical– pressure etc
• Electrical
• Thermal– cold, hot
• Chemical– H+, lactic acid, K+, histamine, bradykinin, acetylcholine, proteolytic
enzymes
– Prostaglandins• these increase the sensitivity (decrease the threshold) for other nociceptive
stimuli
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receptors• there are no specialised receptors
• free nerve endings are sensitive to pain stimuli
• free nerve endings are distributed everywhere• both somatic and visceral tissues• except brain tissue and lung parenchyma
• nociceptors are very slowly adapting type
• different types of nociceptors– some respond to one stimulus– some respond to many stimuli (polymodal)– some may not respond to the standard stimuli (silent nociceptors), they respond
only when inflammatory substances are present
• TRPV1 receptor (capsaicin receptor)– respond to capsaicin, heat, low pH– stimulation leads to painful, burning sensation
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central connections• afferent fibre enters the spinal cord
• synapses in laminae I and II (substantia gelatinosa)• visceral fibres have more diffused distribution (I,V,X)
– (Large Ab fibres termiate in laminae layers III to V)
substantiagelatinosa
Neurotransmitter at the first synapse of the pain pathway is substance P
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Pain
lateralspinothalamic tract
C fibre
substantiagelatinosa
• crosses the midline• ascends up as the lateral spinothalamic tract• pain originating from head and neck region travel through
trigeminal nerve, trigeminal nucleus and supply thalmus • spontaneous firing of trigeminal pathway may result in
“trigeminal neuralgia”
ascending pathway
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lateralspinothalamic tract
thalamus
sensory cortex
C fibre
thalamocorticaltracts
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Pain perception
• This occurs at different levels– thalamus is an important centre of
pain perception• lesions of thalamus produces severe
type of pain known as ‘thalamic pain’
– Sensory cortex is necessary for the localisation of pain
– Other areas are also important• reticular formation, limbic areas,
hypothalamus and other subcortical areas
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Pathophysiology of pain
• Pain sensations could arise due to– Inflammation of the nerves (neuritis)
– Injury to the nerves and nerve endings with scar formation (disk prolapse)
– Injury to the structures in the spinal cord, thalamus or cortical areas that process pain information (spinal trauma)
– Abnormal activity in the nerve circuits that is perceived as pain (phantom limb pain)
– Nerve invasion, for example by cancer (brachial plexopathy)
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Descending pain modulatory system
• several lines of experimental evidence show the presence of descending pain modulatory system
– discovery of morphine receptors– they were known to be present in the brain
stem areas
– discovery of endogenous opioid peptides
• eg. Endorphines, enkephalins, dynorphin
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opioid peptides
• short peptides originally known to be secreted in CNS and later found to be present in GIT etc
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opioid peptides endorphin
• Earliest to discover, present in pituitary
• encephalins - met & leu• widely distributed
• dynorphin• Endomorphine 1 & 2• Pronociceptins
Naloxone is an opioid antagonist. It blocks the actions of opioid
Receptors: mu, kappa, delta, recently discovered ORL1 receptor
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• descending tracts involving opioid peptides as neurotransmitter were discovered
• these were known to modify (inhibit) pain impulse transmission at the first synapse at the substantia gelatinosa
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• first tract was discovered in 1981 by Fields and Basbaum– it involves enkephalin secreting neurons in the reticular
formation– starting from the PAG (periaqueductal grey area) of the
midbrain– ending in the NRM (nucleus raphe magnus) of the medulla– from their ending in the substantia gelatinosa of the dorsal
horn
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midbrain
pons
medulla
spinal cord
periaqueductal
grey nucleus
nucleus raphe
magnus
substantia gelatinosa
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• in the subtantia gelatinosa– enkephalin secreting neuron is involved in
presynaptic inhibition of the pain impulse transmission by blocking substance P release
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substantiagelatinosa
c fibre input
descending inhibitory tract
dorsal horn
substantia
gelatinosa cell
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substance P
enkephalin
Presynaptic inhibition
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Presynaptic inhibition
substance P
enkephalin
pain impulse
blocking of
pain impulse
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• since then various other descending tracts were discovered
• all of them share following common features– involved in brain stem reticular areas
– enkephalins act as neurotransmitters at least in some synapses
– most of these tracts are inhibitory
– midbrain nuclei are receiving inputs from various areas in the cortex, subcortical areas, limbic system, hypothalamus etc
– the ascending tract gives feedback input to the descending tracts
– recently even nonopioid peptides are known to be involved
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• final pain perception depends on activity of the– ascending pain impulse transmitting tracts– descending pain modulatory (inhibitory) tracts
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lateralspinothalamic tract
thalamus
sensory cortex
C fibre
thalamocorticaltracts
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Theories of pain
There is a single pathway for touch and pain
Less intensity produces touch
Increased intensity produces pain
There are two different pathways for touch and pain
Specificity theory
touch pain
Intensity theory
touchpain
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Gate control theory
• This explains how pain can be relieved very quickly by a neural mechanism
• First described by P.D. Wall & Melzack (1965)
• “There is an interaction between pain fibres and touch fibre input at the spinal cord level in the form of a ‘gating mechanism’
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Gate control theory
central control
transmission cell
touch
A fibre
pain
C & A fibres
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Gate control theory
central control
transmission cell
touch
A fibre
when C fibre is stimulated, gate will be opened & pain is felt
pain
C & A fibres
pain is felt
+gate is opened
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Gate control theory
central control
transmission cell
when A & C fibres are stimulated together, gate will be closed & pain is not felt
pain is
not felt
touch
A fibre
pain
C & A fibres
+ -
gate is closed
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Gate control theory
• This theory provided basis for various methods of pain relief– Massaging a painful area – Applying irritable substances to a
painful area (counter-irritation)– Transcutaneous Electrical Nerve
Stimulation (TENS)– Acupuncture ?
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Gate control theory
• But the anatomcal basis for all the connections of Wall’s original diagram is lacking
?
?
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WDR (wide dynamic range cells)
• It is known that some of the second order neurons of the pain pathway behave as wide dynamic range neurons
• they can be stimulated by pain stimuli but inhibited by touch stimuli
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WDR (wide dynamic range cells)
C fibre A fibre
pain &
mech mech
inhibitoryexcitatory
WDR cell
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WDR cells
• have been found in– Spinal cord– Trigeminal nucleus– Brain stem– Thalamus– Cortex
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Modifications to the gate control theory
• this could be modified in the light of enkephalin activity and WDR cells
• inhibitory interneuron may be substantia gelatinosa cell
• descending control is more important
• WDR cells may represent neurons having pain as well as touch input
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referred pain
• sometimes pain arising from viscera are not felt at the site of origin but referred to a distant site.– eg.
• cardiac pain referred to the left arm• diaphargmatic pain referred to the shoulder
– this paradoxical situation is due to an apparent error in localisation
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referred pain - theories
• convergence theory– somatic & visceral structures
converge on the same dermatome
– generally impulses through visceral pathway is rare
– centrally brain is programmed to receive impulses through somatic tract only
– therefore even if the visceral structure is stimulated brain misinterpret as if impulses are coming from the somatic structure
visceral
somatic
second
order
neuron
++++
+
++
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referred pain - theories
• facilitatory theory– somatic & visceral structures
converge on the same dermatome
– stimulation of visceral structure facilitates transmission through somatic tract visceral
somatic
second
order
neuron
++++
+
++
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• hypoxia / pressure / inflating a BP cuff– first affect large A fibres (touch & pressure sense)– then affect A fibres (temperature sense & pricking
pain)– lastly C fibres (burning pain)
• local anaesthetics– first relieve burning pain ( C fibres)– then temperature sense & pricking pain (A fibres)– lastly touch& pressure sensation (large A fibres)
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Pain arising from abdominal viscera
• Mediated by C fibres
• Mainly due to the sensations of distention, muscular contraction, inflammation, hypoxia but not to cutting, tearing, local irritation, burning
• Typically vague, dull, and nauseating
• These structures are innervated by autonomic nerve fibers
• It is poorly localized and tends to be referred to areas corresponding to the embryonic origin of the affected structure
– stomach, duodenum, liver and pancreas referred to upper abdomen
– small intestines, proximal colon and appendix referred to periumbilical pain
– distal colon and GU tract referred to lower abdominal pain
• Peritonitis causes somatic pain
• Diffuse localization of true visceral pain is probably due to the low density of visceral sensory innervation and extensive divergence of the visceral input within the central nervous system
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Capsaicin and vanniloid receptors • Active compound in chilies is capsaicin
• Capsaicin chemically is one of the vanilloids
• Capsaicin receptor is called TRPV1 – (Transient receptor potential vanilloid type 1)
• This receptor is also stimulated by – heat greater than 43°C– low pH
• This receptor is sensitised by prostaglandins and bradykinins
• Upon prolonged exposure to capsaicin TRPV1 activity decreases– this phenomenon is called desensitization– Extracellular calcium ions are required for this phenomenon– This causes the paradoxical analgesic effect of capsaicin
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Cannabinoid receptor
• Cannabis (marijuvana or ganja) causes pain relief
• Cannabis act on cannabinoid receptors found in pain pathway
• There are endocannabinoids as well
• Cannabinoid receptor-related processes are involved in cognition, memory, anxiety, control of appetite, emesis, motor behavior, sensory, autonomic and neuroendocrine responses, immune responses and inflammatory effects
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Neurotransmitters in the CNS
• Excitatory – Substance P– Glutamate (NMDA receptor)– Neurokinin A and B– calcitonin gene-related peptide– vasoactive intestinal polypeptide– Somatostatin– bombesin
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Neurotransmitters in the CNS
• Inhibitory – GABA– Noradrenalin– Serotonin– Enkephalins
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Pain memory • Memory of pain can be more damaging than its initial experience
• Central sensitization – Increased responsiveness of nociceptive neurons in the central nervous system to their normal or subthreshold
afferent input
• Peripheral sensitization – Increased responsiveness and reduced threshold of nociceptive neurons in the periphery to the stimulation of their
receptive fields
• Clinical interventions to blunt both the experience and persistence of pain or to lessen its memory are now applied
• Preemptive analgesia– Pre-emptive analgesia is a treatment that is initiated before the surgical procedure in order to reduce sensitization– Many studies have demonstrated that analgesic intervention before a noxious stimulus or injury is more effective at
averting central sensitization than the same analgesic intervention given after the stimulus