NEUROPHYSIOLOGY OF
PAIN
WHAT IS PAIN?
According to the International Association for the Society of
Pain,
Pain is “an unpleasant sensory and emotional experience
associated with actual or potential tissue damage, or
described in terms of such damage”.
(Loeser, 2011)
AETIOLOGY: WHAT CAUSES PAIN?
• “Pain can be due to a wide variety of
diseases, disorders and conditions
that range from a mild injury to a
debilitating disease”
(Williams, 2011)
ACUTE PAIN
“The terms acute and chronic refer exclusively to the time course of
the pain, irrespective of aetiology”
Acute Pain:
•Usually lasts less than 3 months
•Sudden onset
•Usually know the cause of the pain
•Usually well defined
•Predicable ending (healing)
•Can lead to chronic pain if left untreated
•Examples: cut to the finger, broken bone
CHRONIC PAIN
Chronic Pain:
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Persistent or recurring pain
Continues for more than 3 months
May last for months or even years
Can be difficult to diagnose and treat
Primary goal is not total pain relief but reducing pain relief
• Examples include: arthritis and back pain
CATEGORIES OF PAIN
Another way to categorise pain is on the basis of
origin:
• Nociceptive
• Neuropathic
• Psychogenic
NOCICEPTIVE PAIN
Nociceptive pain is directly related to tissue damage and can be
either external (somatic) or internal (visceral)
External / Somatic
• Most common type of pain
• Can be superficial -in the skin but may extend to the underlying
tissues.
• Usually described as: sharp, shooting, throbbing, burning, stinging
• well defined area
• Usually lasts from a few seconds to a few days
• Examples include: paper cut, sprained ankle
NOCICEPTIVE PAIN
Internal / Visceral (Deep)
Less common and usually more severe
Originates in the walls of visceral organs
Poorly defined area
Described as: deep, aching, pressing or aching
Usually lasts a few days to weeks
Virtually a symptom of all diseases at some point during disease
progression.
• Often associated with feeling sick
• Examples include: Major surgery, labour pain, irritable bowel.
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NEUROPATHIC PAIN
• Injury or disease of the central nervous system
rather than the peripheral tissue.
• May be due to nerve compression,
inflammation or trauma
• Usually lasts between a few months to many
years.
• Difficult to treat due to the lack of knowledge
of the underlying cause.
• Often associated with paraesthesia,
hyperalgesia and allodynia
• Burning, shooting or pins and needles (not
sharp like nociceptive).
PSYCHOGENIC PAIN
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Psychological, psychiatric or psychosocial are the primary causes
Severe and persistent pain
Appears to have no underlying pathology.
Less common now due to medical technology
Pain experienced (Headaches, abdominal pain, back pain) is
indistinguishable from that experienced by people with identifiable
injuries or diseases.
• This kind of pain can be very
frustrating to sufferers and can
interfere with their ability to function
normally.
CLINICAL MANIFESTATIONS
“No two people are likely to experience the same level of pain for a given
painful stimulus”
Pain Tolerance:
The maximum level of pain that a person is able
to tolerate without seeking avoidance of the
pain or relief
What affects Pain Tolerance?
• Fatigue, anger, boredom, apprehension,
sleep deprivation. Alcohol consumption,
medication, hypnosis, warmth, distracting
activities and strong beliefs or faiths.
LOCATION
It is important record a patients pain location to be able to
monitor any changes.
Pain can feel like it is
coming from one part
of the body but in fact it
is another, this type of
pain is called referred
pain.
PATHOPHYSIOLOGY
• Pain is not a disorder or disease.
• A consequential reaction by the body to
noxious stimuli.
• Injury
• Disease
• Pain incorporates
• Cognition
• Emotion
• Behaviour
• Simple pathway to the brain;
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Transduction
Transmission
Perception
Modulation
PATHOPHYSIOLOGY
• Transduction
• Process by which afferent nerve endings participate in translating
noxious mechanical, chemical or thermal impulses into
nociceptive impulses.
• Strong physical stimuli and disease processes cause chemical
release.
• Once activated the chemicals bind to specific receptors.
• chemicals such as bradykinin, cholecystokinin and prostaglandins,
activate or sensitize nearby nociceptors
• Lead to the generation of Action Potentials (AP)
TRANSDUCTION
PATHOPHYSIOLOGY
• Transmission
• 1st Order Sensory Neurons
• Located in the dorsal root ganglia in the posterior of the spinal
cord.
• AP’s are conducted to the CNS primarily via two types of
primary afferent neurons
• A delta Fibres "Epricritic Pain"
• C Fibres "Protopathic Pain"
• 2nd Order Sensory Neurons
• The impulse crosses the spinal cord and ascends to the
thalamus and branches to the brainstem nuclei via central
transmission.
• Messages cross the cord and ascend to the thalamus via the
Spinothalamic pathway, heading to the somatosensory cortex,
the insula, frontal lobes and limbic system.
A-DELTA AND C FIBRES
Nerve fibre
Aδ
C
Appearance
Type of Pain
Epicritic
Protopathic
Information
carried
•Sharp pain
(‘fast pain’)
•Temperature
•Dull pain
(‘slow pain’)
•Temperature
•Itch
Diameter
1-5
(micrometres)
0.2-1.5
Speed of
signal
conduction
0.5-2.0 m/sec
5-35 m/sec
A delta Fibres
• "Epricritic Pain"
• Mechanical message
• Sharp, Fast pain
• Thin Myelinated fibres
increase speed of processing
C Fibres
• "Protopathic Pain"
• Mechanical and Thermal
Stimuli
• Slow, dull, long lasting pain
• Unmyelinated fibres, slower
response
PERIPHERAL TRANSMISSION
Peripheral transmission
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An electron micrograph showing
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large myelinated Aβ
small lightly myelinated Aδ fibres
unmyelinated fibers C Fibres.
SYNAPTIC TRANSMISSION
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Synaptic transmission
• Action potential synapse at
the dorsal horn of the spinal
cord
• Neuroactive excitatory and
inhibitory neurotransmitters
are released
• Lead to generation of
action potentials and
central transmission of pain
signals to higher centres.
PATHOPHYSIOLOGY
• Perception
• When noxious stimuli is recognised.
• Multiple areas of the brain
• 3rd Order Sensory Neurons
• To the higher brain centres of
Limbic system
m
• Frontal cortex, primary sensory cortex of the post central gyrus of
parietal lobe
• Sensory-Discriminative Response
• result of activity in the somatosensory and the insular cortex
• allows the person to identify the type, intensity and bodily location of the
noxious event.
• Affective-Emotional Response
• Mediated by the limbic system.
• Defines the response and associated behaviour.
PATHOPHYSIOLOGY
• Modulation
• Dampening or amplifying pain-related neural signals.
• Descending input from the brainstem influences central nociceptive
transmission in the spinal cord.
• Descending inhibition of nociception through the release of
neurotransmitters such as serotonin, norepinephrine and endogenous
opioids.
• Gate Control Theory (Melzack and Wall, 1965)
• The body can reduce or increase the degree of perceived pain through
modulation of incoming impulses at a gate located in the dorsal horn of the
spinal cord.
• The integration determines whether the gate will be opened or closed,
either increasing or decreasing the intensity of the ascending pain signal.
• Psychological variables in the perception of pain, including motivation to
escape pain, and the role of thoughts, emotions, and stress reactions in
increasing or decreasing painful sensations.