Somatosensation (Chapter 5a)

History:

  • Solomon’s group and inherent need for stimulation   * Individuals become distressed w/in 4-8 hrs, few lasted more than 24 hours

General Sensory Processing:

  • SensorytransductionSensory transduction::when a receptor cells converts the energy of a stimulus into action potentials
  • Each sense has it own specific set of receptor cells
  • Hierarchical organization   * Stimuli → receptor cells → thalamic relay nuclei → primary sensory cortex → secondary sensory cortex → association cortex   * Functional segregation and parallel processing   * Multiple specialized areas at multiple levels - interconnected by multiple parallel pathways
  • Think on a comparative level - how is there variation across species?   * Black fly, panther chameleon, American bald eagle - examples of ocular differences

Labeled Lines:

  • Each nerve input to the brain reports only a particular type of info
  • Distinguishing b/w different types of touch
  • Travel thru the spinal cord and then are fully processed by the brain

Somatosensation:

  • Types of skin -   * Hairy skin   * GlabrousGlabrous::non-hairy skin; high sensitivity (palms of hands, lips, tongues, soles of feet)
  • 2-point sensitivity test   * Glabrous skin - 3mm   * Hairy skin - 2+cm

Somatosensory Receptors:

  • NociceptionNociception::perception of pain and temp; free nerve endings
  • HapticsensationHaptic sensation::perception of fine touch and pressure

Dorsal Column System:

  • Sense of touch neurons are longest in the body - going from longest toe upward thru the spinal cord into the medulla at the brain stem and then to the thalamus
  • Toe → medulla; medulla → thalamus (crisscrosses - if on right side, will move to left); thalamus → somatosensory cortex   * Area of the medulla = decassationdecassation::criss-crosses
  • DermatomesDermatomes::a strip of skin innervated by a particular spinal nerve

Pain:

  • Unpleasant sensory and emotional experience associated w/actual or potential tissue damage
  • 3 dimensions of pain -   * Sensory−DiscriminativeSensory-Discriminative::good pain vs. bad pain; where is the pain?   * Motivational−AffectiveMotivational-Affective::situational motivations and emotions   * Cognitive−EvaluationCognitive-Evaluation::thoughts on the pain, address it as decisions
  • PainParadoxPain Paradox

     1. Adaptiveness of pain - pain is bad, but important for survival   2. Lack of clear cortical representation - remove SI and SII and we still perceive pain   3. Pain can be suppressed - sports injuries, religious rituals, life-threatening situations

Congenital Insensitivity to Pain:

  • Born w/o perception of pain; no increase in HR, blood pressure, or respiration due to pain   * Weak gag reflex, complete lack of corneal reflex
  • The “Human Pincushion”
  • Small cluster in Sweden w/this specific gene

Pain Pathways - Anterolateral System:

  • ADeltafibersA Delta fibers::large in diameter, myelinated axons, fast travel = sharp pain
  • CfibersC fibers::thin, unmyelinated axons, slow travel = dull or throbbing pain
  • Withdrawal reflex to get away from the pain; vocal reflex at the midbrain (exclamation or curse)

Dual Pathways:

  • HapticandPropriocepticPerceptionHaptic and Proprioceptic Perception   * Dorsal Column System   * Enters spinal cord and ascends to the medulla prior to crossing
  • NociceptionNociception   * Anterolateral system   * Synapse in the spinal cord and cross over

Peripheral Mediation of Pain:

  • Inflammation and potential activation of the immune system
  • Neurons are activated - collaterals that come off and release Substance P   * SubstancePSubstance P::further stimulates cells to promote action at the skin level     * Pain fibers release glutamate which goes up the spinal cord and signals the brain

Pain Perception/Long Term Pain:

  • GlutamateGlutamate::mild pain
  • SubstancePSubstance P::intense pain
  • EndorphinsEndorphins::if enough endorphins are produced, Substance P turns down = less pain
  • NeuropathicpainNeuropathic pain::complicated gray area about why pain exists - neurons coded wrong? treat w/opiods?
  • PhantomlimbpainPhantom limb pain::some patients can be treated w/mirror image boxes and are more receptive to prosthesis b/c brain is still maintaining cortical space for the missing limb

Drugs of Interest:

  • NaloxoneNaloxone::opioid antagonist
  • MethadoneMethadone::opioid agonist   * Long acting, decreased euphoria, remove withdrawal effects

Cortical Pathways and Somatosensation:

  • Cingulate cortex; somatosensory agnosias
  • AstereognosiaAstereognosia::inability to recognize objects by touch - focal cortical lesion at area SII, which integrates inputs from finger joints w/tactile info
  • AsomatognosiaAsomatognosia::failure to recognize parts of one’s own body - damage in the right posterior parietal cortex or premotor cortex   * Almost always in a left leg of left arm   * Almost always in stroke victims   * Recovery is weak

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