Sapolsky Chapter 9

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Sapolsky Chapter 9

• Stress and Pain

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Why have pain?

• Why not a neon light on our
  forehead?
• Motivates, as well as informs us.

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The basics of pain perception

• Cellular messengers spill out and trigger pain
  receptors.
• Some receptors only pain (slow, unmyelinated).
• Others paintouch (faster, myelinated).

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The basics of pain perception

• Immune system cells accumulate (scarf up those
  sliced-up cells)
• Mast cells ? histamine ? swelling.
• Information sent to dorsal horn of spinal cord
• Glutamate Substance P transmitters
• Reflex is initiated here.
• Faster than if info had to go to cortex first.

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The basics of pain perception

• Some neurons ascend on opposite side of spinal
  cord to brain.
• Information is sent to thalamus and several areas
  of cortex.
• Somatosensory cortex in parietal lobe location,
  type of pain
• Cingulate cortex emotional content
• Hypothalamus and other areas ? autonomic NS CRH

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Modulation of pain perception

• Melzak Wall Gate theory
• Both fast slow axons excite X neuron?brain

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Sudden pain

To brain
X

Slow pain
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Modulation of pain perception

• Fast neuron also excites the Y interneuron,
• Which inhibits the X neuron.
• The slow pain neuron actually INHIBITS the Y
  interneuron.

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Sudden pain


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To brain
X
Y
-

Slow pain
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Modulation of pain perception

• Note error Sapolski says that a diabetic who
  loses the fast pain fibers loses the ability to
  shut down the Y interneuron.
• (p. 165 in 2nd edition, p. 192 in 3rd edition)
• However, you dont want to shut down the Y
  interneuronyou want to excite it!

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Modulation of pain perception

• Descending axons from the brain to the spinal
  cord, influence pain sensitivity of neuron X.

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Sudden pain
From brain



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To brain
X
Y
-
-

From brain
Slow pain
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Modulation of pain perception

• Patients with views of trees requested less pain
  medication after gall bladder surgery.
• Also, sense of control helped.
• Allodynia pain in response to normal sensory
  stimulus.
• During migraines, after lengthy inflammation

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Stress-induced analgesia

• Stress-induced analgesia
• Henry Beecher (WWII) 80 of civilians and 33
  of soldiers requested morphine for similar
  injuries.
• Exercise after 30 min pain melts away and you
  feel euphoria.
• Animals hot plate test swim or social threat ?
  stress increases time to withdraw foot.

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Stress-induced analgesia

• Endorphins, enkephalins, dynorphins
• (Note opioids vs. opiates)
• Acupuncture (humans and animals)
• Naloxone blocks effects.
• Roger Guillemin beta-endorphin from pituitary
• One source of runners high.
• Not the only source serotonin?
• Relatively brief effect depletion of transmitters

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Summary

• Painful stimulus activates free nerve endings,
  which carry input to spinal cord via either slow,
  unmyelinated fibers (slow pain) or faster,
  myelinated fibers (fast pain).
• The fast fibers stimulate an inhibitory Y
  interneuron, which turns down pain perception.
• The slow fibers inhibit the Y interneuron pain
  continues!

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Summary

• Pain input goes to brainstem, thalamus and
  postcentral gyrus of parietal lobe, where it is
  integrated with other somatosensory information.
• Pain input also goes to anterior cingulate
  cortex, frontal cortex, and hypothalamus for
  emotional interpretation and response.

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Summary

• Peaceful view, sense of control, or stress can
  decrease pain intensity.
• Mediated in part by opioids.
• Stress can also increase pain
• Parietal cortex still responds the same as with
  no stress.
• Emotion-processing areas respond more.
• Anti-anxiety drugs relieve hyperalgesia.