Chapter 34
Disorders of Somatosensory
Function and Pain
Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Three Levels of
Neurons Involved in
Somatic Sensation
• First-order:
detect the
sensation
• Second-order:
in the spinal
cord; transmit
message to
brain
• Third-order: in
the brain
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Sensory Impulses
Sensory impulses
travel up spinal nerves
to the spinal cord
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Second-order Neurons
Carry Sensory
Impulses to the Brain
• Discriminative
pathway
• From spinal nerve
root to medulla
• Cross over to
thalamus on other
side of brain
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Third-order Neurons
Carry Sensory
Impulses to the
Cortex
• Primary
somatosensory
cortex identifies
sensation
• Association cortex
relates sensation
to memories,
other sensations,
etc.
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Question
Which level of neuron carries sensory impulses to the
brain?
a. First-order
b. Second-order
c. Third-order
d. Association
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Answer
b. Second-order
Rationale: Second-order neurons travel from the spinal
nerve root to the medulla oblongata (brain stem or
hindbrain), and cross over to the thalamus on the
opposite side of the brain.
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Discussion
How does an “itch” sensation reach the brain and
how is it interpreted?
• What was the:
– Receptor type?
– First-degree neuron type?
– Spinal nerve?
– Pathway to the brain?
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Pathway of Pain
Impulses
• Anterolateral pathway
– Paleospinothalamic
tract
º To reticular
activating system
º Affects arousal,
mood, attention
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Pathway of Pain
Impulses (cont.)
• Anterolateral pathway
– Neospinothalamic
tract
º To thalamus and
parietal cortex
º Allows localization,
identification of
pain
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Question
Tell whether the following statement is true or false.
The paleospinothalamic tract allows you to identify the
location of your pain.
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Answer
False
Rationale: The neospinothalamic tract allows you to tell
where your pain is (helpful when you describe symptoms
to a health care provider); the paleospinothalamic tract
activates the reticular activating system (controls
sleep/wake cycles and allows you to maintain a state of
consciousness).
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What Makes Pain Different?
• Specificity theory
– Special pain receptors detect pain (nociceptors)
• Pattern theory
– Sensory receptors create pain signals when
stimuli are too strong
• Gate control theory
– Pain is carried by distinct fibers in the spinal cord
• Neuromatrix theory
– The brain identifies pain
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Nerve Fibers that Carry Pain Impulses
• A-delta fibers
– Large, myelinated fibers
– Impulses travel quickly; “fast pain”
– Release glutamate at the synapse with the spinal
neurons
• C fibers
– Small, nonmyelinated
– Impulses slower; “slow pain”
– Release glutamate and substance P
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Periaqueductal Gray Region
• Endogenous
analgesic
center
• Stimulated by
opioids
• Can send
nerve impulses
to inhibit other
neurons in the
pain pathway
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Kinds of Pain
• Cutaneous
• Deep somatic
• Visceral
• Referred
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Altered Pain Sensitivity
• Hyperpathia: continued stimulation causes pain
• Paresthesias: spontaneous, unpleasant sensations
• Dysesthesia: distortions of somesthetic sensation
• Hypalgesia: reduced pain sensation
• Analgesia: absence of pain
• Allodynia: pain after non-noxious stimulus
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Question
Your patient has been given morphine following minor
surgery. What effect will the morphine have?
a. Parasthesia
b. Dysesthesia
c. Analgesia
d. All of the above
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Answer
c. Analgesia
Rationale: Morphine is the prototype opioid. CNS
depressants/narcotics are commonly used to relieve the
sensation of pain (analgesia = absence of pain).
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Neurogenic
Inflammation
tissue
damage
inflammatory
mediators
stimulate
nociceptors
impulses run up
C fibers
dorsal nerve root
reflex
inflammatory mediators move
back down and are released into
tissues
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Causes of Neuropathic Pain
• Pressure on nerve
• Physical injury to neuron
• Chemical injury to neuron
• Infection of neuron
• Ischemia
• Inflammation
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Neuropathic Pain Syndromes
• Trigeminal neuralgia (tic douloureux)
• Postherpetic neuralgia
• Complex regional pain syndrome
• Phantom limb pain
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Types of Headaches
• Migraine
• Cluster
• Tension-type
• Temporomandibular joint pain
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