Pain physiology

1. PAIN PHYSIOLOGY
Prof. Dr. Rashid Mahmood

2. Prof. Dr. Rashid Mahmood
PAIN Physiology

3. Objectives
• Goal/Aim: To give the understanding of the
physiology of pain
• Specific Objectives: At the end of the lesson
student will be able to:
• Classify the types of pain.
• Describe the pain pathways
• Explain the mechanism of referred pain.
• List examples of referred pain
• Describe the Analgesia system.
© Prof .Dr. Rashid Mahmood
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4. Lesson Contents
• Definition of Pain
• Types of Pain
• Pain Receptors
• Stimuli For Pain Receptors
• Mechanism of Stimulation of Pain Receptors
• Pain Pathways
• Analgesia System
• Methods for relieving Pain
• Referred Pain
• Visceral Pain
• Pain Insensitive Viscera
• Clinical Abnormalities of Pain
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5. MCQ1: Fibers involved in the
sensation of fast sharp pain are:
A. Alpha
B. A-delta
C. A-beta
D. C fibers
E. A gamma
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6. MCQ2: Fast pain is transmitted
at a velocity of about
A. 0.5-2m/sec
B. 2-4m/sec
C. 12-30m/sec
D. 40-60m/sec
E. 70-80m/sec
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7. MCQ3: First relay station of pain is:
A. Basal ganglia
B. Medulla
C. Pons
D. Spinal Cord
E. Thalamus
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8. MCQ4: The neurotransmitter of
fast pain is:
A. Bradykinin
B. Glutamate
C. Histamine
D. Prostaglandins
E. Substance – P
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• Definition of Pain: Unpleasant sensation
caused by tissue damage.
Purpose:
Protective
Sherrington: “the physical adjunct of an imperative protective reflex”
International Association for the Study of Pain (IASP) Definition: “An
unpleasant sensory and emotional experience associated with actual
or potential tissue damage, or describe in terms of such damage”.

10. Types of Pain
1) Fast Pain:
Sharp, Pricking, Acute, Electric, Physiological
0.1 Sec
Skin
2) Slow Pain:
Burning, Aching, Throbbing, Nauseous,
Chronic, Pathological
0.1 Sec, Increase Slowly
Skin + Deeper Tissues
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Nociception
• Process by which pain information is
carried from the peripheral sensory
receptors in the skin and viscera to the
cerebral cortex through network of
neuronal relays

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Pain Receptors (NOCICEPTORS)
• Free nerve ending
• Location:
Superficial layers of skin
Periosteum, arterial walls, joint surfaces, falx and
tentorium of cranial vault
Other deep tissues
• Non-Adapting

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Stimuli For NOCICEPTORS
Types of NOCICEPTORS
1. Mechanical NOCICEPTORS
2. Thermal NOCICEPTORS
1. Warmth Receptors
2. Cold Receptors
3. Chemically Sensitive NOCICEPTORS
Bradykinin, Serotonin, Histamine,
K+, Acids, Acetyl-Choline, Proteolytic enzymes.
4. Polymodal NOCICEPTORS
Combinations of these stimuli

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Mechanism of Stimulation of Pain
Receptors
1. Tissue Damage
1. Heat
2. Chemical Pain
2. Tissue Ischemia
1. Lactic Acid
2. Bradykinin
3. Proteolytic Enzymes
3. Muscle Spasm
1. Direct Stimulation
2. Ischemia

15. Chemical Mediators in response
to tissue damage
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Fig. 8-5, Ganong, 24th Edition

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Pain Pathways
• Two Pathways (Dual Pathway)
1. Fast-Sharp Pain Pathway
(Neospinothalamic Tract)
2. Slow-Chronic Pain Pathway
(Paleospinothalamic Tract)

17. ASSESSMENT Q.1
•Define Pain
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Peripheral Pain Fibers
(Fast & Slow Fibers)
• Fast-Sharp Pain
Signals:
Mechanical & Acute
Thermal Pain
Small type Aδ fibers
Myelinated
6-30 m/sec
• Slow-Chronic Pain
Signals:
Chemical Pain,
Persisting Mechanical
& Thermal Pain
Small type C fibers
Unmyelinated
0.5-2 m/sec
Double Pain
Sensation

19. Peripheral Pain Fibers (Fast &
Slow Fibers)
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Characteristic
Fast-Sharp Pain
Signals
Slow-Chronic
Pain Signals
Stimulus Type
Mechanical &
Acute Thermal
Pain
Chemical Pain,
Persisting Mechanical
& Thermal Pain
Fiber Type Small type Aδ fibers Small type C fibers
Myelination Myelinated Unmyelinated
Conduction Speed 6-30 m/sec 0.5-2 m/sec

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Carried by three neurons
• 1st,
• 2nd , and
• 3rd order neurons

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1st order neuron
• Fast-Sharp Pain Pathway
• (Neospinothalamic Tract)
 Terminates in Lamina I
(Lamina Marginalis)
 2nd order neurons start
here
 Neuro-Transmitter:
• Glutamate
• Slow-Chronic Pain
Pathway
• (Paleospinothalamic Tract)
 Terminates in Lamina II &
III (Substantia Gelatinosa)
 Interneurons
 Lamina V
 2nd order neurons start
here
 Neuro-Transmitters:
• Substance P: Slow action
• Glutamate: Immediate
action
Carries impulse
from nociceptor to spinal cord
Enters dorsal horn
Rises in Tract of Lissauer

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Transmission of both "fast-sharp" and "slow-
chronic" pain signals into and through the
spinal cord on their way to the brain.
1st order
neuron

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2nd order neuron
• Carries impulse from spinal cord to thalamus/Brain
Stem
• Fibers cross immediately to the opposite side
through Anterior Commisure
• Anterolateral Spinothalamic Tract
• Termination of fibers:
Fast-Sharp Pain Pathway:
Mainly: Thalamus (Ventro-
basal Complex)
A few fibers: Brain Stem
(reticular Formation)
Slow-Chronic Pain Pathway :
A few fibers: Thalamus
(Intralaminar Nuclei)
Mainly: Brain Stem
1. Reticular Nuclei of medulla,
Pons & Mesencephalon
2. Tectal area of Mesencephalon
3. Periaquedubtal grey region

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2nd order neuron

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3rd order neuron
• Carries the impulse
from the thalamus to
the Primary Sensory
Cortex according to the
topographical
representation of the
body
• When the Primary
Sensory Cortex is
stimulated, the person
becomes aware of the
pain

26. ASSESSMENT Q.2
• Why a sudden painful sensation gives
double pain sensation?
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27. Take Home Points
• Pain is an unpleasant sensation caused by
tissue damage. It may be Fast Pain or Slow
Pain
• Pain Receptors, called NOCICEPTORS are
Free nerve ending & are Non-Adapting
• Stimuli For NOCICEPTORS may be
Mechanical, Thermal or Chemical
• Pain is transmitted to brain via Two Pathways
(Dual Pathway): Fast-Sharp Pain Pathway
(Neospinothalamic Tract) & Slow-Chronic
Pain Pathway (Paleospinothalamic Tract) 27
© Prof .Dr. Rashid Mahmood

28. Learning Resources
• Guyton and Hall (Text book of physiology),
13th Edition
• Ganong (Text book of physiology), 2`5th
Edition
• Masood Ahmed Khan (Neurophysiology
for Medical students, 3rd edition)
• Lecture Notes on Neurophysiology by
Prof. M. Tariq (2nd edition)Sherwood’s
Physiology, 9th Edition
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29. MCQ1: Fibers involved in the
sensation of fast sharp pain are:
A. Alpha
B. A-delta
C. A-beta
D. C fibers
E. A gamma
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B

30. MCQ2: Fast pain is transmitted
at a velocity of about
A. 0.5-2m/sec
B. 2-4m/sec
C. 12-30m/sec
D. 40-60m/sec
E. 70-80m/sec
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C

31. MCQ3: First relay station of pain is:
A. Basal ganglia
B. Medulla
C. Pons
D. Spinal Cord
E. Thalamus
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D

32. MCQ4: The neurotransmitter of
fast pain is:
A. Bradykinin
B. Glutamate
C. Histamine
D. Prostaglandins
E. Substance – P
© Prof .Dr. Rashid Mahmood
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B

33. Thank You
Questions?
Comments?
drrashid62@gmail.com
rashid.mahmood@rmi.edu.pk