The Somatosensory System: Thermoception and Nociception

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The Somatosensory System Thermoception and
Nociception
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Week 10 Outline

• Temperature Perception (thermoception)
• Pain Perception (nociception)

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Thermoception

• Function maintain thermal equilibrium
  especially important for proper brain functioning
• Thermoreceptors
• Cold receptors (free nerve endings, Krause end
  bulbs) (cold)
• range of response 15-35C (59-95F), peak at 30C
  (86F)
• Lie near surface of skin
• Warm receptors (free nerve endings, ruffini
  cylinders)
• range of response 30-48C (86-118F), peak at
  44C (111F)
• lie deeper under skin
• Crude localization no overlap of receptor types
  (Figure 16.19)

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Factors affecting Temperature Perception

• Sensation of Temperature depends on thermal
  conductivity, the rate at which heat is
  transferred to or from the body
• Conduction smooth surfaces more efficient that
  textured surfaces
• Convection movement of air or water increases
  rate of heat transfer
• Radiance
• Thermal adaptation
• Skin temperature is normally about 33C (91F)
• both warm and cold fibers will adapt to sustained
  temperatures over a range of 16-42C (61-108F)
• Physiological zero (Figure 16.20)
• temperature range of 2-4C (5-9F) surrounding
  currently adapted skin temperature
• Neither warm nor cold fibers will respond to
  temperatures in this range
• Paradoxical Cold
• Heat Grill (Figure 16.21)

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Pain Perception

• What is Pain?
• Pain is our friend the adaptive nature of pain
• Studies of individuals who do not experience pain
  
• Cohen, Kipnes, Kunkle, Kubzansky, 1955
• Jewsbury, 1951
• Baxter and Olszewski (1960)

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Peripheral Pain Mechanisms

• Nociceptors from latin nocere to injure
• Free nerve endings
• pain perception is largely independent of
  perception of pressure and temperature

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Peripheral Pain Mechanisms

• Peripheral mechanisms for pain
• Bradykinin released by injured cells
• Bradykinin enhances synthesis and release of
  prostaglandin
• Prostaglandin increases the sensitivity of the
  nociceptor free nerve endings to histimine
• Pain Remedies
• Aspirin interferes with prostaglandin synthesis
• Antihistimines reduce the levels of histimine
  that stimulate the free nerve endings

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Two Pain Pathways

• Slow Pathway C fibers ? spinothalamic pathway
• Fibers synapse at reticular formation (in
  brainstem) then project from brainstem to
  thalamus
• Carries persistent, nonlocalized, slower onset
  dull pain or burning sensations
• Fast Pathway A-delta fibers ? spinothalamic
  pathway
• Fibers synapse directly in the thalamus
• Carries transient, highly-localized, fast onset
  bright or prickly pain sensations (first pain)

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Bottom-up Influences on Pain

• Skin stretching
• Tactile stimulation
• Counterirritation
• bite the bullet
• Chronic Alcoholism and nicotine poisoning
• Degradation of myelinated peripheral nerve fibers
• Disrupts perception of fast pain

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Top-down Influences on Pain

• Phantom Limb Pain
• Descending Pain Pathways
• Mental State expectations, attitudes, attention,
  motivation, emotions, and cognition can affect
  the intensity and quality of pain

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Top-down Influences on Pain

• Anxiety and Pain
• Beecher (1972)
• Morphine reduces pathologically produced pain
  that is accompanied by anxiety, but
• Morphine does not reduce experimentally-produced
  pain that is unaccompanied by anxiety
• Anecdotal Evidence
• Surgical patients
• Surgical vs. battlefield pain

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Top-down Influences on Pain

• Stress and Pain
• Jessel and Kelly (1991)
• Pain thresholds in animals are raised by stress
• Adaptive in a stressful situation pain is a
  hindrance rather than a help
• Placebos and Pain
• Weisenberg (1977) 35 of patients with
  pathological pain get relief from taking a placebo

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Top-down Influences on Pain

• Culture and Pain
• Asian Indian Hook-swinging ceremony (Melzack)
• Keim (1981) Canadian Native Americans and spinal
  bone grafts

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Facial Nerves and Pain

• Trigeminal nerve
• Largest of 12 cranial nerves
• Three major branches
• Ophthalmic nerve
• Sensory information (tactile, thermoception,
  nociception, proprioception) from green areas,
  nasal mucosa,and frontal sinuses
• Maxillary nerve
• sensory information from pink areas, nasal
  mucosa, palate, ethmoid and sphenoid sinuses
• Mandibular nerve
• Sensory input from yellow areas, floor of the
  mouth, and anterior 2/3 of tongue
• Motor control of muscles involved in biting,
  chewing, and swallowing

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Neural Mechanisms of Pain

• Gate Control Theory
• Melzack and Wall (1965)
• Perception of pain mediated by a gate located
  in the dorsal horn of the spinal cord

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Experimental Evidence for the Gate

• Selective inactivation of L-fibers results in
  greater pain perception from noxious stimuli
  (Price, Hi, and Dubner, 1977)
• Phantom Limb Pain may result from reduced L-fiber
  input (Melzack, 1970)

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Experimental Evidence for the Gate

• Stimulation-produced Analgesia (SPA)
• Electrical stimulation of mid-brain in rats
  produces an analgesic effect (Reynolds, 1969)
• Acupuncture may be a form of SPA
• Bottom-up counter-irritation effect from
  stimulation of L-fibers
• May also release endogenous pain
  suppressors?endorphins

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Endorphins and Pain

• Endorphins neurotransmitters that act as
  endogenous (naturally-occurring) morphine-like
  substances
• Endorphins bind to same receptor sites in brain
  stem as opiates
• SPA works best when endorphin sites are
  stimulatedmay release endorphins into the
  nervous system (Hosubuchi et al., 1977)

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Endorphins and Pain

• Concentration of endorphins is generally less for
  people suffering from chronic pain (Akil et al.,
  1978)
• Opiate inhibitors (e.g., naloxone) decrease the
  analgesic effects of acupuncture, SPA, and
  placebos
• Stress-induced analgesia may result from
  increased release of endorphins during stress