Brain Growth and Neural Development

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Brain Growth and Neural Development
Reflexes
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Dynamic Action System

• Small changes during development lead to major
  changes in structure and functioning later on
• Prenatal and postnatal
• Changes attributed to plasticity
• What does this mean?
• Example ocular dominance
• 7 Primary Processes in Neural Dev

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Neurogenesis (step 1)

• Proliferation of cells/prenatal
• Neurons produced in neural tube during prenatal
  development
• Glial cell production starts before
  birth-continues beyond birth
• forms a network that the neurons use to migrate
• Increases efficiency

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Migration (2)

• Cells move through passive displacement (pushed
  out by newly forming cells)
• Active displacement- along the glial cell
  structures
• Plasticity- the degree to which a developing
  structure or behavior is modifiable due to
  experience.
• Example cell destined for visual area misses
  mark
• ends up in auditory area
• differentiates

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Axon Growth (3)

• Axons grow toward specific targets
• Guidance occurs as a result of neurotrophic
  factors-
• family of proteins - growth and survival of the
  neuron
• Chemical attraction or repulsion causes growth
  cone of axon to change direction

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Maturation (4)

• Neurons are formed and migrating before birth (by
  end of 2nd trimester)
• Neurons are functional but not optimally (at
  adult levels)- we see inconsistent firing
  patterns
• Activity pattern changes with experience to
  become more efficient and consistent
• See Kaylin video

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Synaptogenesis (5)

• Increase in synaptic connections
• Dendritic branching begins increasing shortly
  before birth
• These high levels are associated with infant
  relflexes
• E.g. visual synapses
• _at_10 of adult levels _at_ birth
• _at_ 8 months, synapses are at max levels
• _at_ 11-12 years, adult levels (50-60 of max level)

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Decrease in of synapses (6)

• Big increase, then gradually decrease in number
  after birth (8 mo)
• Apoptosis- programmed cell death- predetermined
  lifespan of the cell related to neurotrophic
  factors

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Decrease in of Synapses (6) -Pruning

• Pruning- increased activity in specific areas
  associated with a decrease of the neuronal
  network.
• decrease in density occurs at the same time as
• regression of infantile reflexes
• acquisition of specific abilities
• seems to be a critical mass of synapses required
  for onset of specific skills acquisition.
• as skills are refined? pruning

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Myelination (7)

• Responsible for increased wt size of brain
• Neurons in place after birth- axons become coated
  with fatty substance after birth (Glial Cells).
• Glial cells
• Taste, smell, touch are myelinated partially or
  fully at birth
• vision- process complete sometime around 5-8
  months
• language processing pathways
• begin around 12 months
• rapid expansion of myelin sheath around 15-18
  months- Result?
• myelination associated with acquisition of new
  skills- speed of transmission
• brain growth spurt- last trimester to end of 2nd
  year
• Additional growth spurt 5-8 years

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MRI Studies
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Brain Growth

• Size
• Functionally developed at birth
• _at_ birth brain weighs ¾ - 1 lb (25 of adult size)
• _at_ 6 months, 50 of adult size (1.5-2 lbs)
• _at_ 2 years 75 of adult size (2.25-3 lbs)
• Fetal Brain Growth

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Why such growth?

• What accounts for increased size?
• Addition of glial cells?Myelination of the axons
• Shape
• As size increases- shape changes
• Deeper folds
• More convolutions
• Increases the surface area