Different types of nociceptors have been identified that respond to mechanical, heat and chemical stimuli, or any combination of these stimuli.
Prof. Ashraf Abdou
An unpleasant sensory and
emotional experience associated
with actual or potential tissue
damage or described in terms of
International Association for the Study of Pain
Function of Pain
Pain protects humans by warning of occurrence of
biologically harmful processes
reflexes, regulated at the level of spinal cord, protect
by removing a body part away from danger
associated emotional arousal, experienced as distress
or fear, may also motivate a person to move away from
a painful stimulus
Fear of pain can also prevent a person from moving,
which in turn promotes healing of the injury resulting in
pain may elicit an empathic, comforting, and health
promoting behavior in people observing a person in pain.
Age and perception of pain
Children and the elderly may experience or express
pain differently than adults
Infants in the first 1 to 2 days of life are less
sensitive to pain (or they simply lack the ability to
verbalise the pain experience).
A full behavioural response to pain is apparent at 3 to
12 month of life
Older children, between the ages of 15 and 18 years,
tend to have a lower pain threshold than do adults
Major Categories of Pain
Classified by inferred pathophysiology:
1. Nociceptive pain (stimuli from somatic
and visceral structures)
2. Neuropathic pain (stimuli abnormally
processed by the nervous system)
Acute pain: lasts less than 6 months,
subsides once the healing process is
Chronic pain: involves complex
processes and pathology. Usually involves
altered anatomy and neural pathways. It is
constant and prolonged, lasting longer
than 6 months, and sometimes, for life.
Types of Somatic Sensations
Transmission/Perception of Pain
Four specific parts of
the nervous system
transmit pain signals
from the periphery to
the higher centers of
the dorsal horn
the ascending tracts,
Robinson(1997), Journal of Hand Therapy
receptors, are the
first order neurons of
Free nerve endings
signals in response
to harmful stimuli.
Robinson(1997), Journal of Hand Therapy
Cell bodies of the nociceptors reside in the dorsal
root ganglia (DRG).
Nerve fibers leaving the DRG bifurcate and send one
branch to the periphery and the other branch to the
dorsal horn (DH).
The peripheral fibers conduct pain signals from the
skin, muscles, fascia, vessels, and joint capsules to
The sensory peripheral fibers, have been classified into three
types based on their diameter, myelination and conduction
velocity: A (with four subtypes – α, β, γ and δ) B and Cfibers.
A-δ fibers (Myelinated) conduct fast pain (a sensation
experienced immediately after an injury that indicates location of
C-fibers (Un-myelinated) conduct slow pain (follows sharp pain
and can be characterized as a dull, throbbing ache with poor
The brain first perceives the sensation of pain
The thalamus, sensitive cortex :
• Parts of thalamus, brainstem and reticular formation:
- identify dull longer-lasting, and diffuse pain
The reticular formation and limbic system:
- control the emotional and affective response to pain
Because the cortex, thalamus and brainstem are
interconnected with the hypothalamus and autonomic
the perception of pain is associated
with an autonomic response
Thalamus contains nerve centers responsible for vision, hearing
reflexes, equilibrium and posture. It also relays pain signals to
the cerebrum. Crude sensation reaches consciousness in the
thalamus, the cerebral cortex is responsible for the higher thought
processes such as emotion and interpretation.
Regulation of Pain: Accentuation
Passive transmission of noxious stimuli cannot explain how
people experience pain. Pain experience can be explained
by an active process.
This active process includes several regulatory
mechanisms that participate in attenuating or accentuating
the perception of a noxious stimulus.
An accentuated pain experience can be associated
with factors such as edema, fear, anxiety, and release
of endogenous chemicals that sensitize nerve endings.
Spinal cord level mechanisms explain an accentuated pain
2) wind-up, and
3) central sensitization.
It occurs when normally
innocuous stimuli begin to
Following intense, repeated,
or prolonged stimulation, or if
inflammation is present,
the threshold for primary
afferent nociceptors is
the frequency of firing is
higher for all stimuli.
Example: cloths become
painful in case of neuropathy
Low frequency repetitive stimulation of C-fibers
produces a gradual increase in the discharge
Example: frequently touching the hot plate becomes painful
In this state there are
augmented responses to input and
enlarged receptive fields.
Input from areas that previously did not activate the WDR neuron
now evoke a prominent response, and
Low threshold stimulation is able to drive the neurons.
Wind-up is elicited by any prolonged or intense C-fiber
It results in an
responsiveness of the
horn neurons resulting
conduction of pain
signals to the brain.
injury, an area of
adjacent to the damaged
tissue can evoke pain
Regulation of Pain: Attenuation
2 classic examples of pain
First, after injuring a hand, a
person may shake it vigorously
to reduce pain sensation.
Second, an athlete, although
injured during a game, may not
feel injury related pain until end
Regulatory mechanisms that
attenuate pain act at four levels of
the dorsal horn
the descending fibers (from
periaqueductal gray, raphe
nuclei, and locus ceruleus),
hormonal system (
Theory of pain production and modulation
ON GATE CONTROL THEORY (created by Melzack and Wall)
• According to this theory, nociceptive impulses are
transmitted to the spinal cord through large A-δ and
small C- fibers. These fibers create synapses in the SG
• The cells in this structure function
as a gate, regulating
transmission of impulses to CNS
Stimulation of larger nerve fibers (A- α, A- β} causes
the cells in SG to
"close the gate".
closed gate decreases stimulation of T-cells (the
2nd afferent neuron), which decreases transmission of
impulses, and diminishes pain perception
Abnormal processing of the impulses
either by the peripheral or central nervous
May be caused by injury (amputation and
subsequent phantom limb pain), scar
tissue from surgery (back surgery high
risk), nerve entrapment (carpal tunnel), or
damaged nerves (diabetic neuropathy)
Painful Mononeuropathies and Polyneuropathies
Trigeminal and other CNS neuralgias
Pappagallo M. 2001.
The Concept of Total Pain
Analgesic effects separate from anti-depressant
Amitriptyline: most studied, but most side effects
Nortriptyline & desipramine: better tolerated, less
SSRIs: little evidence of analgesic effect.
inhibit both norepinephrine and serotonin reuptake
efficacy in neuropathic pain syndromes or pain
associated with depression (duloxetine
[Cymbalta®], venlafaxine [Effexor®])
Agents for neuropathic pain
Start low, go slow
Watch for side effects
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