HORMONES AND BEHAVIOR Psychology

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• Development and Plasticity of the Brain
• Development of the Brain (5.1)
• growth and differentiation
• apoptosis
• pathfinding by axons
• effects of experience
• Recovery of Function After Brain Damage (5.2)
• causes of human brain damage
• recovery after brain damage
• phantom limbs
• therapy

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Development of the Brain

• our brain is not fully developed at birth
• there is significant brain growth during early
  childhood
• different parts of the brain develop at
  different times

• object permanence (Piaget) depends on the
  prefrontal cortex
• prefrontal cortex matures by 7.5 - 12 months
  (Goldman-Rakic)

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Growth and Differentiation of the Vertebrate Brain
The human CNS begins to form at 2 weeks of
gestation.
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Types of Neural tube defects

• Spina Bifida----neural tube doesnt completely
  close (usually in the lumbo-sacral region)
• Anencephaly---absence of cranium and brain
• Hydrocephaly---defect in ventricles

These conditions can be detected in utero
during mid-pregnancy by an increased
concentration of alphafetoprotein in the amniotic
fluid or by ultrasound.
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• at birth, brain weighs 350 g
• at 1 y, brain weighs 1000 g
• adult, brain weighs 1200-1400 g

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Stem cells

• Cells lining the ventricles of the brain.
• These cells can differentiate into any type of
  cell.
• Neurotrophic factors signal what the cells will
  eventually become.

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Growth and Development of Neurons

• Proliferation the process by which new cells are
  produced.
• cells lining the ventricles divide early in
  development
• some of these cells will become neurons and
  glia
• Migration the process by which cells move to
  their final destination.
• requires the precise chemical environment
• genetic anomalies environmental insults
  interfere with this process
• Differentiation process by which cells form
  axons and dendrites.
• axon develops first
• dendrites develop when cell has finished
  migrating
• Myelination the process by which glial cells
  produce myelin sheaths.
• spinal cord, hindbrain, midbrain, forebrain
• can continue for years decades
• Synaptogenesis The formation of synapses

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An Example Loss of motor neurons in the SC of a
developing fetus.

• number of motor neurons is greatest 11 wks
• drops steadily until 25 wks
• this is when most motor neuron axons make
  synapses with muscles
• axons that fail to make synapses die

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Determinants of Neuron Survival

• Nerve Growth Factor (NGF) a neurotrophic protein
  that promotes the survival and growth of the
  axon.
• each neuron has a suicide mission
• its axon must make contact with the appropriate
  postsynaptic cell
• NGF prevents the suicide
• Neurotrophin a chemical that promotes the
  survival and growth of a neuron.
• enhances cell survival/growth during early
  development
• promotes axonal branching
• increase growth of damaged axons
• Apoptosis the process of programmed cell death.
• apoptosis is not necrosis
• halted by NGF

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Apoptosis
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Chemical Gradients Guide Axon Growth

• chemical attractants steer the axon to grow in
  the correct direction
• a growing axon follows a path of cell surface
  molecules, attracted by some chemicals but
  repelled by others
• this steers the axon in the right direction
• upon reaching the correct location axons will
  make tentative connections with many postsynaptic
  cells
• over time, the postsynaptic cell will strengthen
  some and reject others (neural Darwinism)

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Pathfinding by Axons

• Axons find their targets by following chemical
  pathways (Sperry, 1943)
• cut the optic nerve of a newt
• it grew back and re-innervated the optic tectum
• newt regained normal vision in that eye

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• cut the optic nerve
• rotated the eye by 180
• dorsal retina fibers grew back to the portion
  of the tectum responsible for vision in the
  dorsal retina
• ventral retina fibers grew back to the portion
  of the tectum responsible for vision in the
  ventral retina
• how was Sperry able to determine that the nerve
  fibers grew back to the correct places???

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There is Competition Among Axons!!
Motor neuron axons - muscle fibers
SNS ganglia receive multiple synapses
Each muscle fiber typically strengthens only 1
synapse
Each dendrite typically strengthens only 1
synapse
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Fine Tuning by Experience

• Experience affects dendritic morphology
• rat studies thicker cortex improved
• performance in learning tests
• bird studies specialization of dendritic
  spines
• Generation of new neurons
• olfactory receptors
• stem cells (undifferentiated) in the interior
  of
• the ventricle
• new cells in the hippocampus and cerebral
  cortex

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Plasticity after Brain Damage

• Many people show behavioral recovery after brain
  damage.
• Brain damage can occur in many ways.
• In young people, it most often occurs via a
  closed head injury. In older people, a stroke or
  hemorrhage.
• Age at the time of injury is important.
• There are various mechanisms of recovery after
  brain damage.

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Causes of Human Brain Damage
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Mechanisms of Recovery after Brain Damage

• Learned adjustments in behavior.
• Regrowth of axons.
• Collateral sprouting.

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Reorganized Sensory Representations Phantom Limbs
there is brain reorganization after amputation...
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This can lead to referred sensations and pain
amputation
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A Method of Relieving Phantom Limb Pain