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Plasiticity and Learning (9 Organization Principles of Cortical Circuits…

- - - - Neocortex of all mammals is built of a common circuit
  - - - 80% of neruons
      - are spiny and have a prominent apical dendrite
      - Glutamate neurotransmitter
      - excitatory
    - - 20% of neurons
      - do not have densely spiny dendrites
      - GABA neurotransmitter
      - inhibitory
      - axon of most cells does not project out of their own cortical area
    - - spiny but lack of apical dendrite
      - exist only in middle layers of certain sensory cortical areas
      - axon of most cells does not project out of their own cortical area
  - - - commonly interpreted as evidence for random connections
      - however, statistical averaging says nothing about specificity of neuron-to-neuron connections; quantification is needed
      - there are studies suggested specific connections exist while Peters's rule still holds in overall statistics
  - - - standard cable theory predicts that the most efficient site for inhibitor synapses is he proximal region around cell body, where indeed dominated by inhibitory neurons
      - inhibitory synapses at distal regions are found to be more efficient to quench distal excitatory signals
  - - - long-distance connections in low in number
      - feedforward
        
        originate from neurons in superficial layers
        
        contact to middle layer of the target area
      - feedback
        
        contact outside the middle layers
        
        originate from deep layers
      - number of neurons involved falls exponentially with distance
      - lateral connections
        
        originate from all layers
        
        contact to all layers
  - - - thalamic synapses are weak
      - there is strong recurrent excitation within and between layers
      - excitation and inhibition are linked in tandem to remain in balance and to prevent overexciting the circuit
    - - express the functional relationships bw the excitatory and inhibitory neurons in the different cortical layers
      - show how the inputs to a local region of cortex from the sensory periphery via the thalamus or other cortical areas are integrated
- - - - limits on brain size place constraints on the number and density of neurons
      - allometric scaling law
        white matter volume growing faster than gray matter volume as brain size increases
      - spatially close neurons tend to be densely and mutually interconnected, conserving wiring cost
      - connection probabilities are reduced as physical distance increases
    - - metabolic cost in maintaining brain function and its rest state
      - cause evolutional pressure to increases energy efficiency, neuronal coding strategy that minimize energy use with action potentials, and sparse coding
  - - - each module corresponds to a set of densely interconnected nodes while connections bw modules are sparse
      - dense connections within modules and weak interconnections bw then for a structural basis for functional specialization
      - network hubs
        specific subset of nodes dedicates to communication paths bw modules; act as points of convergence and divergence for intermodular signal
- - - - main idea
        
        e.g. derease in GABAergic inhibition and increase spontaneous and driven neural activity
        
        neuroplasticity is responsible for compensation mechanisms during natural aging
    - - plasiticity within a system exhibits both enhancements and deficits in processing
      - e.g. visual dorsal pathway exhibits high degree of plasticity compared to ventral path, visual tasks related to dorsal path can be enhanced (as in congenital deaf person) or impaired (as in dyslexic person)