NeurosciencePain II

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Neuroscience-Pain II

• Pain processing through the dorsal horn and brain
• Vahn Lewis, UTDB

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Brainstem neurobiology

• Entry of primary afferents to dorsal horn
• Laminar projection
• Trigmenial is homologous
• Interneurons 50 of cells
• Many neurotransmitter systems
• Brainstem hyperalgesia
• depolarization of pools of spinal neurons

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Primary Afferent Terminal Zones in Dorsal Horn
Primary afferents project to specific layers in
the spinal cord.
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Fine Structure of the Dorsal Horn
The dorsal horn is populated with
many interneurons, these may play a role in
pain processing.
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Dorsal Horn Response Patterns to Different Acute
Noxious Stimuli
Formalin
Heat
Pinch
The nature of the response depends on the
stimulus type.
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Summary of Dorsal Horn Pain Modulators
The dorsal horn receives peripheral, local and
descending modulation involving a variety of
neurotransmitter systems
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Consequences of brainstem organization

• Temporal and spatial summation
• Impact of injury discharge
• Phantom pain
• Windup
• Allodynia
• Referred pain
• Heterosynaptic hyperalgesia

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Definition 1Temporal Summation in Teeth
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Definition 2Spatial Summation in Teeth
Spatial Summation in Paired Tooth Stimulation
0.63
0.9
1.01
0.61
0.73
.94
1.06
MMultiple Tooth Threshold
ASum of Individual Thresholds
M/AIndex of Convergence
M/A of .63 mean only 63 of the current
needed for a single tooth pain threshold
is needed when paired teeth are stimulated.
After Mumford and Newton 1974
pairteet.mgx/val/10/30/95
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Definition 3 Injury discharge

• Injury induces a strong signal that can
  activate the dorsal horn of the spinal cord for
  several minutes.
• The important sensory transmitter glutamate is
  released.
• Glutamate acting on NMDA receptors can lower
  sensory thresholds leading to a centrally
  mediated hyperalgesia
• Simultaneously released Substance P also mediates
  the hyperalgesia

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The Impact of Central Hyperalgesia on Central
Pain Processing
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Definition 4Phantom Limb Pain

• Phantom pain is pain that seems to arise from a
  part of the body that has been amputated.
• The incidence of phantom pain is greater when the
  amputation induces a large injury discharge or
  the limb is in pain at the time of amputation.
• Phantom pain suggests that pain related
  hyperalgesia can produce a long-lasting,
  pathological, reorganization dorsal horn function.

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Definition 5Windup

• 3 HZ or faster repeated pain stimuli or
  prolonged spontaneous C fiber activation produces
  an increasing pain signal.
• Windup can be blocked by blocking NMDA type
  glutamate receptors located in the dorsal horn.

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Definition 64. A-beta Allodynia
(Light touch fibers mediating pain)
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Definition 7Referred pain
Pain from visceral organs is often referred to
peripheral sites which are distant from the pain
source
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Divergence of Trigeminal Input
Mumford has mapped the wide distribution of
the afferent terminal of the Trigeminal nerve.
This extensive fiber distribution provides
opportunities for referred pains and extensive
pain spread.
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Convergence in the Trigeminal Nucleus

• Afferents from various types of tissue provide
  converging inputs onto the trigemino-thalamic
  cells in the deep subnucleus caudalis
• Injury of multiple tissues can produce pain
  additively.

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Definition 8. Heterosynaptic Hyperalgesia
Hyperalgesia Induces a
Receptor Field Increase
Pinching here
activates cell
Touch here
activates
cell
Expanded receptive field
after inflammation.
Responses recorded from a trigeminal subcaudalis
multi-m
odal neuron. Inflammation induced with a
injection of mustard oil into the deep masseter
muscle.
Fom Hu et al, Pain, 8(1992)53-60
hupic.mgx/val/10/30/95
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Referred Pain from the Lateral Pterygoid Muscle
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Referred Pain From Medial Pterygoid Muscle
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Referred Pain from the Temporalis Muscle
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Referred Pain from the Masserter Muscle
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Example of pain referral from neck to head.
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Parietal vs Visceral Pain
In some cases pain from a single injury can be
sensed as multiple pains. Inflamed appendix pain
can produce both a deep burning pain and a
referred skin pain
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Pain Referred From Teeth
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Peripheral vs Central Hyperalgesia
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Importance of Pain Prevention

• Pain causes reflex release of sensitizing
  chemicals in the peripheral wound area
• Sensitizing chemicals can recruit additional pain
  receptors
• Damage to peripheral nerve may lead to chronic
  pain
• Injury discharge activates brainstem dorsal
  horn cellular pools.
• Brain stem or spinal cord sensitization broadens
  hyperalgesic fields-heterosynaptic effects
• Pain discharge can induce excitotoxic changes.
• Phantom pains increase when pain is present
  during amputation

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Referred Pain from the Sternocleidomastoid Muscle