Basal ganglia: Selection

Modular architecture for multifunctional systems
1. Including the brain
  1. Largely independent parallel processing functional units
    1. Each with
      1. Specific functional objectives
      2. Specialised sensory input
      3. Specialised behavioural output
    2. The selection problem
      1. At any point in time which system should be permitted to direct motor output (behaviour)?
        Competing functional (motivational) systems
        Spatially distributed
        Processing in parallel
        All act through final common motor path
      2. Sensorimotor version
        Sub-cortical visual system
Basal ganglia
1. Fundamental processing unit
2. Essential component of all vertebrate brains
3. Evolutionary conservation
  1. “The basal ganglia in modern mammals, birds and reptiles (i.e. modern amniotes) are very similar in connections and neurotransmitters, suggesting that the evolution of the basal ganglia in amniotes has been very conservative.”
  2. Implication
    1. If the basal ganglia have been conserved to the degree we think they have...
      1. Then the problems they were evolved to solve were just as much problems for early vertebrates as they are for us today
      2. Sub-cortical systems could provide important clues
4. Summary
  1. Selection is a generic problem common to all vertebrates and applies to sensorimotor/cognitive and motivational/affective options
  2. Competing behavioural options relayed to different territories of basal ganglia as 'bids' for expression
  3. Selective disinhibition within basal ganglia loops returns a 'winner takes all'
  4. Sophisticated cortical and basic sub-cortical options sometimes in competition
Architecture
1. Re-entrant loops
2. Pre-cortical loops through the basal ganglia
3. Striatum
  1. Ventromedial/dorsolateral gradient
  2. Microarchitecture common across functional territories
    1. External inputs
      1. Cerebral cortex
      2. Limbic system
      3. Brainstem
        Via thalamus
      4. Input functions
        Cognitive
        Affective
        Sensorimotor
4. Why loops?
  1. An architectural solution to the selection problem
  2. Critical features
    1. Multiple competing functional systems
      1. Different classes of competition resolved in different functional territories
    2. Segregated loops
      1. Phasic excitatory nput
      2. Tonically active inhibitory output
    3. Selective disinhibition
      1. Is a mechanism for selection
Diseases
1. Parkinsons
  1. Degeneration of ascending DA projections
  2. L-dopa (DA precursor) treatment of choice for many years
  3. Akinesia
    1. Inability to disinhibit (select) any channel
  4. Bradykinesia
    1. Partial ability to disinhibit
2. Schizophrenia
  1. DA antagonists - Antipsychotic
  2. DA agonists - induce/potentiate psychosis
  3. Also ADHD and tourettes
    1. Dysfunctional 'soft switching'
    2. Winners win small, losers lose small
    3. Vulnerable to interrupt
3. Drug addiction
  1. Most non-theraputically used drugs either
    1. Increase DA transmission directly
      1. Amphetamine
      2. Cocaine
    2. Or interact with it indirectly
      1. Nicotine
      2. Heroin
  2. Also OCD
    1. Dysfunctional 'hard switching'
    2. Winners win big, losers lose big
    3. Resistant to interrupt
      1. Perservation
4. Variable switching between winning and losing channels
5. Disorders of selection
  1. Phobias
    1. Trigger stimuli harmless - uncontrollable fear
  2. Anxiety-panic attacks
    1. Situations not dangerous - incapacitating anxiety
  3. PTSD
    1. Situation not dangerous - trigger stimuli
  4. Addictions
    1. Explicit knowledge of detrimental effect - powerless in face of sensory stimuli
  5. Head vs heart
    1. Situations where we should know better!

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Basal ganglia: Selection

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	Criado por Kristi Brogden quase 10 anos atrás