General Definitions of Pain

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General Definitions of Pain Term sometimes given
to strictly mental processes. Animals aversive
reaction to strong stimulation Physical damage
to the body, i.e. burn or cut. Previous injury -
could be described as painful, unbareble
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• Pain and Pleasure
• Intensive Theory Weber vs Erb and Goldscheider
• Pattern Theory. Nafe (1929)
• Mullers Theory of Specific Nerve Energies Von
  Frey proposed four distinct sets of somatic
  sensory receptors touch, warm, cold and pain.

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Aberrant pain

• Inflammation and hyperalgesia
• primary hyperalgesia
• secondary hyperalgesia - allodynia where a
  previously non-noxious stimulus induces pain
• Referred Pain
• Pain and Pathology

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In man stimulation of the afferents conducting
at gt 40 m/sec produce the sensation of touch,
while lt 25 m/sec (Ad) fibers indicate pain,
increase intensity of stimulation and include C
fibers then subjects report intense pain.
Idea of two pains First pain (pricking) which is
associated with Ad fibers and Second pain
(burning and diffuse) associated with C
fibers. The Ad fibres can be mechanosensitive
only or mechano-heat sensitive.
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Pressure block. Large fibers are blocked first,
small last. Touch sensation is lost first and
pain last. Local Anaesthetic. Small fibers are
blocked first and large last. Here pain is lost
first and touch last.
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The polymodal fibers in monkey respond to heat at
temperatures greater than 45 deg C also to
mechanical stimulation and to noxious chemical
stimulation. One of the most used substances to
produce chemical pain is capsaicin, the active
substance in chilli.
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Silent - initially silent nociceptors. These
are reported in muscle, joints and visceral
tissues and most recently in the skin responding
to sustained pressure on the skin. C fibers
that are initially unresponsive become
responsive to stimulation. What is significant
about these observations is that the C fibers
that do respond initially give an initial
response and then adapt after time. But the
initially silent fibers dont respond to start
with and then they respond after about --- (30
min ) with a constant response. The important
psychophysical observation is that subjects
dont initially report pain but then after
period they report pain.
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The polymodal fibers in monkey respond to heat at
temperatures greater than 45 deg C but on
repeated application there can be a lowering of
threshold and an increase in activity with
repeated stimulation. This can be mimicked by
formalin injection as a model for primary
hyperalgesia. Sensitization has two components. -
peripheral (local to the nerve endings ) and
central. There is some evidence that NGF is
pivotal in both aspects. The NGF depletes the
mast cells which release histamine and 5-HT.
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Central pathways

• How does touch information get to the CNS?
• How does pain information get to the CNS?

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Central Sensitization might be due to the
transport of NFG to the CNS where it acts to
produce increased sensitivity. The central effect
is likely to be mediated by EAAs l-glutamate or
l-asparate and and is blocked by NMDA
antagonists. One hypothesis is that it is the
accumulation of intracellular Ca that mediates
increases in NO, arachodonic acid and PKC. The
hypothesis (unspecified) is that these
intracellular messengers produce long-term
alterations in the membrane properties leading
to increased excitability.
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General Definitions of Pain Term sometimes given
to strictly mental processes. Animals aversive
reaction to strong stimulation Physical damage
to the body, i.e. burn or cut. Previous injury -
could be described as painful, unbareble
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• McGill Pain Questionnaire
• Sensory-discriminative quality (throbbing,
  gnawing,
• shooting )
• 2. Motivational-affective (tiring, sickening,
  fearful )
• 3. Cognitive evaluative quality (no pain,
  ,mild, intense,
• excruciating )

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• cingulate and insula cortex

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• cingulate and insula cortex

Mild discomfort of 47 deg water verses more
discomfort Illusory pain from alternating warm
and cool bars activates Cingulate.
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Descending pain modulation

• Naturally occurring opiates
• endorphin
• enkephalins

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Placebo effect
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