Touch and Pain

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Touch and Pain

• Biological Foundations

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Somatosensation
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• Somatosensory receptors vary in complexity and
  stimuli that they respond to
• Pacinian corpuscle detects sudden displacements
  or high-frequency vibrations on the skin
• Meissners corpuscles
• elaborate neuronal endings detect sudden
  displacement and low frequency vibrations on skin

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Somatosensation
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• free nerve endings detect pain, warmth and cold
• Ruffini endings detect stretch of skin
• Merkels disks detect indentation of skin

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Somatosensation
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• Input to the spinal cord and the brain
• touch information from head enters CNS through
  cranial nerves
• below the head, information enters via 31 spinal
  nerves connecting to 31 dermatomes

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Somatosensation
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• sensory pathways to cortex remain separate
• Ex two parallel strips respond to light touch,
  two others respond mostly to deep touch and
  movement of the joints and muscles
• somatosensory cortex receives input from the
  contralateral side of the body

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Theories of Touch

• Von Freys specificity Theory
• Pressure Meissner corpuscles
• Pain free nerve endings
• Warmth Ruffini endings
• Cold Krause end bulbs

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Figure 7.14
James W. Kalat
Biological Psychology, 8th Edition
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• Figure 7.14  Dermatomes innervated by the 31
  sensory spinal nerves. Areas I, II, and III of
  face are not innervated by the spinal nerves, but
  instead by three branches of the fifth cranial
  nerve. Although this figure shows distinct
  borders, the dermatomes actually overlap one
  another by about one-third to one-half of their
  width.

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Touch

• Most likely pathway to Somatosensory Cortex
• receptors
• transduce environmental energy
• Pacinian corpuscle touch receptor
• elliptical shape
• large size

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Touch

• Fibers
• A-beta fast
• A-delta moderate (temp and pain)
• C-fibers slow conducting (temp and pain)

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Touch A Fibers

• A-beta fibers to spinal cord
• 2 ascending tracts
• Fasciculus grancilis (lower part of body)
• Fasciculus cureatus (upper body)
• Spinal cord to brain
• The spinal cord tract synapse and cross over in
  the medulla
• Information goes through the trigenimal nerve to
  the thalamus
• Relayed to the somatosensory cortex

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Pain
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• Transmission
• for moderate pain axons release glutamate
• stronger pain axons release glutamate and
  substance P
• mice without substance P cannot detect severe
  injury

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Pain
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• Associations
• No association with one particular stimulation
• All pain has the potential to cause tissue damage
• Often have emotional reactions
• Pain centers
• No single center in the brain for pain

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Pain
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• Psychology of Pain
• Environmental factors
• Culture
• Previous experience
• Mental state influence
• Up to 35 of patients with pathological pain get
  relief from placebos

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Pain
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• Receptors
• A-delta fibers
• Short, sharper prickling pain
• C fiber endings
• Longer, buring, dull pain

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Pain
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• Neural pathway
• Fibers enter dorsal horn
• Terminate in laminae I and V of the spinal cord
• Ascend in the anterolateral system
• Fiber release Substance P
• Pain decreases

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Pain
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• Pain blocking
• Pain information can be blocked or transmitted
• Substantia gelatinosa (spinal cord)
• Laminae II and III (spinal cord)

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Pain
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• Involved brain areas
• Reticular formation
• Spinoreticular tract
• Arousal
• Limbic system
• Spinomescencephalic tract
• Emotional
• Thalamus
• Posterior nuclear group
• Dull, burning pain
• Ventrobasal complex
• Sharp, prickling pain
• Cortical area
• Cingulate gyrus

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Pain
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• Gate Control Theory
• Melzak and Wall (1965)
• Proposed that pain perception is controlled by a
  neural circuit
• Two fibers
• S fibers small in diameter, increases activity
  of T-cell and perception of pain
• L fibers large diameter, decreases activity of
  the T-cell and perception of pain

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Pain
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• Gate Control Theory
• Substantia gelantinos
• closes the gate
• Send inhibitory messages to T-cell
• Activated by L-fibers (A-fivers) to close gate
• Deactivated by S-fibers (C-fibers) to open gate

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Pain
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• Opioid mechanisms in brain reduce pain
• endorphins
• neurotransmitters met-enkephalin and
  leu-enkephalin
• bind to opiate receptors
• endorphins are stimulated by pain, especially
  inescapable pain, sex, long-distance running and
  thrilling music

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Pain
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• Opioid mechanisms in brain reduce pain
• supports gate theory of pain that non-pain
  stimuli can reduce pain
• Endorphins released in the periaqueductal gray
  area results in blocking release of substance P,
  reducing pain

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Figure 7.15
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• Figure 7.15  Synapses responsible for pain and
  its inhibition. The pain afferent neuron releases
  substance P as its neurotransmitter. Another
  neuron releases enkephalin at presynaptic
  synapses the enkephalin inhibits the release of
  substance P and therefore alleviates pain.

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Chronic Pain
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• Definition
• Situations in which pain persists long after the
  precipitating injury has healed
• Phantom Limb Pain amputees
• Causalgia bullet wounds
• Neuralgia peripheral nerve infections

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Chronic Pain
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• Pain management
• Suppression
• Stimulations in two areas results in analgesia
• Periaqueductal gray in midbrain
• Nucleus raphe magnus in medulla
• Opiates
• Stimulate periaqueductal grapy
• Inhibits a-delta and C-fibers

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Chronic Pain
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• Pain management
• Medications
• SSRIs in lower doses
• Dual action antidepressants in lower doses
• Elavil
• Acupuncture
• Works through endorphins
• Needles active L-fibers and close the gate

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Painful Heat
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• Body has special heat receptors that respond to
  burns or high heat above 43 degrees centigrade
• capsaicin stimulates heat receptors and causes
  neurons to release substance P, increasing pain
• but, capsaicin leaves you temporarily insensitive
  to pain because neurons are quickly depleted of
  substance P

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Pain and Emotion
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• Hurt is an emotion
• we can ignore serious injury at times, e.g.,
  soldier in battle
• placebo, drug with no effect, can relieve pain
  anesthesia
• also, analgesic is more effective when you know
  it is being given

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Pain and Emotion
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• Cingulate cortex reacts to emotional aspect of
  pain, not the sensation
• painful stimulus to skin results in response
• no response to pin prick when person is told it
  will not hurt
• expectation of pain leads to response to
  moderately warm stimulus
• when damaged in rats, they will react to pain on
  foot but will not learn to avoid the place where
  it was received

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Sensitization, Pain Control and Itching
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• Damaged tissue increases number of sodium gates
  in nearby receptors to magnify pain
• facilitates activity at capsaisin receptors,
  increasing pain
• anti-inflammatory drugs, e.g., ibuprofen,
  decrease pain by reducing the release of
  chemicals from damaged tissues
• Morphine for pain control
• very effective it reducing serious pain
• post-surgical use recommended

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Sensitization, Pain Control and Itching
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• Itch
• caused by release of histamines when skin is
  irritated
• inhibitory relationship with pain, e.g., when
  novocaine wears off, you feel itch but face is
  still numb