This document discusses pain perception and classification. It describes how pain is categorized into somatic and visceral pain. Somatic pain can be superficial or deep, while visceral pain is sometimes referred to other sites innervated by the same spinal segment. The document also summarizes the characteristics of different types of primary afferent fibers that conduct pain signals, including Aδ fibers and C fibers. Finally, it outlines ascending and descending pain pathways in the central nervous system and various mechanisms of pain modulation.

2. PAIN
Prof. Ashraf Husain

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Classification of pain
• Pain perception is affected by the context
in which it is experienced.
• Pain is arbitrarily categorized into somatic
- superficial and deep - and visceral pain.
• Visceral pain is sometimes referred to
another site.
• Structures to which the pain. is referred
are innervated by the same spinal segment
as the origin of the pain.
• Pain may arise from phantom limbs or be
triggered by inappropriately mild
stimulation in abnormal conditions.

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CHARACTERISTICS OF PRIMARY
AFFERENT FIBRES
A δ Fiber:
• Myelinated -
• Diameter fine 2 - 5 μm
• 12 - 30 m/sec. conduction velocity
• Terminated at I and V layer
• Fast pain, rapid, pricking and well localized
• Neurotransmitter - Glutamate
• 20% pain conduction

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• Non-Myelinated
• Diameter less than 2 f.lm .-0.5 to 2 m/s
conduction velocity
• TeJrninate in I and n layers
• Slow, diffuse, dull, aching
• Neurotransmitter - P-Substance
• 80% of pain conduction
CHARACTERISTICS OF PRIMARY
AFFERENT FIBRES
C – Fiber:

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TYPES:
1. HIGH TRHESHOLD
MECHANORECEPTOR(HTM) NNERVATED BY
- TIHN MYELINATED A FIBERS
2. POLYMODAL NOCICEPTORS(PMN)
INNERVATED BY UNMYELINATED C FIBRES
NOCICEPTOR: PAIN RECEPTOR

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DAMAGE AND INFLAMMATION
RELEASE CHEMICAL MEDIATORS WICH ACTIVATE
OR
SENSITISE THE RECEPTOR ENDINGS
CYTOKlNES BRADYKININ, PROSTAGLANDIN, P - SUBSTANCE
RESULTS IN TRANSDUCTION
CONDUCTION OF NERVE IMPULSE

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DORSAL HORN GATING MECHANISM

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A MODAL OF “GATING” OF PAIN

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Ascending
Pathway

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Descending
Analgesic
Pathway

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ASCENDING AND DESCENDING PAIN PATHWAYS

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Neurotransmitters in
Analgesic
Pathway

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Referral of pain from the internal
organs
• Meninges Back of head and neck
• Heart Central chest arms
(usually left), neck,
occasionally abdomen.
• Trachea Behind sternum
• Diaphragm Shoulder tip
• Oesophagus Behind sternum
Cont…
Organ Site of reffered pain

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• Stomach, duodenum Upper abdomen,
epigastrium
• Small bowel, pancreas Around umbilicus
• Large bowel, bladder Lower abdomen
above pubic bone
Organ Site of reffered pain

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VISCERAL PAIN

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VISCERAL PAIN PATHWAY
Pain - Aδ and fibers
Travel with autonomic afferent
Spinal cord
(Dorsal Horn)
Lat. spinothalamic tract
Thalamus
Somatosensory Cortex

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• Poorly localized
• Associated with nausea and autonomic
disturbance
• Often referred to another part of the body
• Cutting, crushing are not painful when applied
to viscera
• Pain is caused by distension, ischemia and
inflammation
CHARACTERSITICS OF VISCERAL PAIN

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• Hyperalgesia: Excessive Pain
• Allodynia: Pain caused by any other sensation
e.g. touch will cause pain.
• Muscular Pain: Less blood flow in the muscles
(ischemia).
• Stress analgesia: Mild degree of pain is not felt if
the other part of the body has excessive pain.
• Causalgia: Burning pain.
TERMS FREQUENTLY USED

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Thalamic Syndrome
.Obstruction of the thalmogeniculate branch of
the posterior cerebral artery Affects posterior
thalamic nuclei.
. Prolonged severe pain.
TERMS FREQUENTLY USED

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SITES & MECHANISM OF PAIN RELIEF

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Trans Coetaneous
Electrical Nerve Stimulation
(TENS)

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