1. Physiology of pain
Prof. Vajira Weerasinghe
Professor of Physiology, Faculty of Medicine, University of Peradeniya &
Consultant Neurophysiologist, Teaching Hospital, Peradeniya
Uploaded to moodle
www.slideshare.net/vajira54
2. Topics covered in the lecture
1. Definition of pain
2. Different terms describing states of pain perceptions
3. Nociceptors
4. Pain stimuli and the role of prostaglandins
5. Ascending pathway
6. Central projections of pain pathway
7. Substance P
8. Descending pain modulatory system
9. Opioid peptides
10. Gate control theory of pain
11. Neurotransmitters
12. Referred pain
13. Methods of pain relief
3. 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
• McGill Pain Questionnaire
4. 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
5. What is pain?
• Pain is
– subjective
– protective
– and it is modified by developmental, behavioural, personality and cultural
factors
• It is a symptom
• Associated signs are crying, sweating, increased heart rate,
blood pressure, behavioural changes etc
• Multidimensional nature of pain
6. Measurement of pain
• It is difficult to describe pain although we know
what it is
• It is difficult to measure pain
– visual analogue scale (VAS) is used
7. Dual nature of pain
• Fast pain
– acute
– pricking type
– well localised
– short duration
– Thin myelinated nerve
fibres are involved (A
delta)
– Somatic
• Slow pain
– chronic
– throbbing type
– poorly localised
– long duration
– Unmyelinated nerve fibres
are involved (c fibres)
– Visceral
8. 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 (pain pathways)
•Neuropathic pain
9. Pain terminology
International Association for the Study of Pain
• Hyperaesthesia
– Increased sensitivity to stimulation, excluding the special senses
(increased cutaneous sensibility to thermal sensation without pain )
• Allodynia
– Pain due to a stimulus that does not normally provoke pain
• Hyperalgesia
– Increased pain from a stimulus that normally provokes pain
• Neuralgia
– Pain in the distribution of a nerve or nerves
• Analgesia
– Absence of pain in response to a normally painful stimulus
• Anaesthesia
– A loss of sensation resulting from pharmacologic depression of nerve
function or from neurological dysfunction
• Paraesthesia
– An abnormal sensation, whether spontaneous or evoked
10. Pain terminology
International Association for the Study of Pain
• Nociceptive pain
– Pain that arises from actual or threatened damage to non-neural tissue
and is due to the activation of nociceptors
• eg. Burns, fractures, injury
• Neuropathic Pain
– Pain caused by a lesion or disease of the somatosensory nervous
system
• eg. Sciatica, neuropathy
• Nociplastic pain
– Pain that arises from altered nociception without clear evidence of actual
or threatened tissue damage causing the activation of nociceptors or
evidence for disease or lesion of the somatosensory system causing pain
• eg. Chronic back pain, fibromyalgia, irritable bowel syndrome
11. Transduction and perception
• Transduction
– Process of converting noxious stimulus to action
potentials
• Perception
– Central processing of nociceptive impulses in order
to interpret pain
12. Stimuli
• Physical
– pressure etc
• Electrical
• Thermal
– cold, hot
• Chemical
– H+, lactic acid, K+, histamine, bradykinin, serotonin, leucotrines,
acetylcholine, proteolytic enzymes, capsaicin
– Prostaglandins (PGE2)
• Cannot directly stimulate nociceptors
• Increase the sensitivity of nociceptors for other stimuli (decrease the
threshold)
13. Receptors
There are no specialised receptors
Pain receptors are called nociceptors
A sensory receptor that is capable of transducing and
encoding noxious stimuli (actually or potentially tissue
damaging stimuli)
Nociceptors are free nerve endings
Free nerve endings are distributed everywhere
both somatic and visceral tissues
except brain tissue and lung parenchyma
14. Receptors
• 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
• Capsaicin receptor (TRPV1 receptor)
– Respond to capsaicin, heat, low pH
– Stimulation leads to painful, burning sensation
15. Nerve pathways carrying pain signals to
the brain
• Pain signals enter the spinal cord
• First synapse is present in the dorsal horn of
the spinal cord
• Cross over to the other side
• Then the second order neuron travels through
the lateral spinothalamic tracts
17. central connections
• afferent fibre enters the spinal cord
• synapses in laminae ii,iii
– substantia gelatinosa
substantia
gelatinosa
Neurotransmitter at the first synapse of the
pain pathway is substance P
• Acute pain : glutamate
• Chronic pain: substance P
• Pain inhibitory neurotransmitters: enkephalin, GABA
20. 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
21. 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)
22. Descending pain modulatory system
• several lines of experimental evidence
show the presence of descending pain
modulatory system
– stimulus produced analgesia (Reynolds)
– stimulation of certain areas in the brain stem was
known to decrease the neuronal transmission along
the spinothalamic tract
– discovery of morphine receptors
– they were known to be present in the brain stem
areas
– discovery of endogenous opioid peptides
• eg. Endorphines, enkephalins, dynorphin
24. opioid peptides
• short peptides originally known to be secreted
in CNS and later found to be present in GIT etc
25. opioid peptides
∀ β endorphin
• Earliest to discover, present in pituitary
• Enkephalins or encephalins - met & leu
• widely distributed
• Dynorphin
• Endomorphine 1 & 2
• Pronociceptins
Receptors: mu, kappa, delta, recently discovered ORL1
receptor
26. • 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
27. • 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
28. • in the subtantia gelatinosa
– enkephalin secreting neuron is involved in
presynaptic inhibition of the pain impulse
transmission by blocking substance P release
32. • 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
33. sensorycortex
C fibre
Final pain perception
depends on activity
of the
Ascending
pain impulse
transmitting
tracts
Descending
pain modulatory
(inhibitory) tracts
34. 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
touch
pain
35. 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’
36. Gate control theory
When pain fibre is stimulated, gate will be opened & pain is felt
pain
pain is felt
+
gate is
opened
37. Gate control theory
When pain and touch fibres are stimulated together, gate will be
closed & pain is not felt
pain is
not felt
touch
pain
+ -
gate is
closed
38.
39. 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 ?
40. Gate control theory
• But the anatomcal basis for all the connections
of Wall’s original diagram is lacking
?
?
41. 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 are responsive to several somatosensory
modalities (thermal, chemical and mechanical)
• They can be stimulated by pain but inhibited by touch
stimuli
42. WDR (wide dynamic range cells)
C fibre A fibre
pain &
mech mech
inhibitory
excitatory
WDR cell
43. WDR cells
• have been found in
– Spinal cord
– Trigeminal nucleus
– Brain stem
– Thalamus
– Cortex
44. 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
45. 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
46. 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
++
++
+
+
+
47. 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
++
++
+
+
+
