Pinel basics ch07
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Pinel basics ch07 Pinel basics ch07 Presentation Transcript

  • Chapter 7 Development of the Nervous System From Fertilized Egg to You
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  • Neurodevelopment
    • Neural development – an ongoing process, the nervous system is plastic
    • Complex
    • Experience plays a key role
    • Dire consequences when something goes wrong
  • The Case of Genie
    • What impact does severe deprivation have on development?
    • At age 13, Genie weighed 62 pounds and could not chew solid food
    • Beaten, starved, restrained, kept in a dark room, denied normal human interactions
    • Can the damage be undone?
  • The Case of Genie
    • Genie’s story is often cited for what it told us about language development (she only uses short utterances), but it also illustrates the impact of abuse on all aspects of behavior
      • No response to temperature extremes
      • Unable to chew
      • Extremely inappropriate reactions (‘silent tantrums’)
      • Easily terrified
    • How can neurodevelopment explain this?
  • Phases of Development
    • Ovum + sperm = zygote
    • Cells then multiply and
      • Differentiate
      • Move and take their appropriate positions
      • Make the needed functional relations with other cells
    • Developing neurons accomplish these things in 5 phases
  • Induction of the Neural Plate
    • A patch of tissue on the dorsal surface of the embryo
    • Development induced by chemical signals from the mesoderm (the “organizer”)
    • Visible 3 weeks after conception
    • 3 layers of embryonic cells
      • Ectoderm – outermost, mesoderm – middle, endoderm - innermost
  • Induction of the Neural Plate
    • Part of induction is inhibition of bone morphogenetic proteins that suppress neurodevelopment
    • Totipotent – earliest cells have the ability to become any type of body cell
    • With the development of the neural plate cell destinies become specified – cells are multipotent – able to develop into any type of mature nervous system cell
  • Stem cells
    • Neural plate cells are often referred to as stem cells. Stem cells:
      • seem to have an unlimited capacity for self-renewal
      • can develop into different mature cell types (totipotent)
    • As the neural tube develops specificity increases, resulting in glial and neural stem cells (multipotent)
  •  
  • Neural Proliferation
    • Neural plate folds to form the neural groove which then fuses to form the neural tube
    • Inside will be the cerebral ventricles and neural tube
    • Neural tube cells proliferate in species-specific ways – 3 swellings at the anterior end in humans will become the forebrain, midbrain, and hindbrain
  • Migration
    • Once cells have been created through cell division in the ventricular zone of the neural tube they migrate
    • Migrating cells are immature, lacking axons and dendrites
    • Radial migration – towards the outer wall of the tube
    • Tangential migration – at a right angle to radial migration, parallel to the tube walls
    • Most cells engage in both types of migration
  • Migration
    • Two types of neural tube migration
      • Radial migration – moving out – usually by moving along radial glial cells
      • Tangential migration – moving up
    • Two methods of migration
      • Somal – an extension develops that leads migration, cell body follows
      • Glial-mediated migration – cell moves along a radial glial network
  •  
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  • Aggregation
    • the process of cells that are done migrating aligning themselves with others cells and forming structures.
    • Cell-adhesion molecules (CAMs) – aid both migration and aggregation
    • CAMs found on cell surfaces, recognize and adhere to molecules
  • Axon Growth and Synapse Formation
    • Once migration is complete and structures have formed (aggregation), axons and dendrites begin to grow
    • Growth cone – at the growing tip of each extension, extends and retracts filopidia as if finding its way
    • Chemoaffinity hypothesis – postsynaptic targets release a chemical that guides axonal growth – but this does not explain the often circuitous routes often observed
  • Axon growth
    • Mechanisms underlying axonal growth are the same across species
    • A series of chemical signals exist along the way
    • Such guidance molecules are often released by glia
      • chemoattractants attract growing axons
      • chemorepellants repel them
    • Adjacent growing axons also provide signals
  • Axon growth
    • Pioneer growth cones – the 1 st to travel a route – follow guidance molecules
    • Fasciculation – the tendency of developing axons to grow along the paths established by preceding axons
    • Topographic gradient hypothesis – seeks to explain topographic maps
  • Synaptogenesis
    • Formation of new synapses
    • Depends on the presence of glial cells – especially astrocytes
    • High levels of cholesterol are needed – supplied by astrocytes
    • Chemical signal exchange between pre and postsynaptic neurons is needed
    • A variety of signals act on developing neurons
  • Neuron Death and Synapse Rearrangement
    • ~50% more neurons than are needed are produced – death is normal
    • Neurons die due to failure to compete for chemicals provided by targets
      • Increase targets > decreased death
      • Destroy some cells > increased survival of remaining cells
      • Increase number of innervating axons > decreased proportion survive
  • Life-preserving chemicals
    • Neurotrophins – promote growth and survival, guide axons, stimulate synaptogenesis
      • Nerve growth factor (NGF)
    • Both passive cell death (necrosis) and active cell death (apoptosis)
    • Apoptosis is safer than necrosis – “cleaner”
  • Postnatal Cerebral Development in Human Infants
    • Postnatal growth is a consequence of
      • Synaptogenesis
      • Myelination – sensory areas and then motor areas. Myelination of prefrontal cortex continues into adolescence
      • Increased dendritic branches
    • Overproduction of synapses may underlie the greater plasticity of the young brain
  • Development of the Prefrontal Cortex
    • Believed to underlie age-related changes in cognitive function
    • No single theory explains the function of this area
    • Prefrontal cortex plays a role in working memory, planning and carrying out sequences of actions, and inhibiting inappropriate responses
  •  
  • Effects of Experience on Neural Circuits
    • Neurons and synapses that are not activated by experience usually do not survive – “use it or lose it”
    • Humans are uniquely slow in neurodevelopment – allows for fine-tuning
    • How do nature and nurture interact to modify the early development, maintenance, and reorganization of neural circuits?
  • Early Studies of Experience and Neurodevelopment
    • Early visual deprivation:
      • fewer synapses and dendritic spines in 1° visual cortex
      • deficits in depth and pattern vision
    • Enriched environment:
      • thicker cortices
      • greater dendritic development
      • more synapses per neuron
  • Competitive Nature of Experience and Neurodevelopment
    • Monocular deprivation changes the pattern of synaptic input into layer IV of V1
    • Altered exposure during a sensitive period leads to reorganization
    • Active motor neurons take precedence over inactive ones
  • Effects of Experience on Topographic Sensory Cortex Maps
    • Cross-modal (involving at least 2 different senses) rewiring experiments demonstrate sensory cortex plasticity – with visual input, auditory cortex can see
    • Change input, change cortical topography - shifted auditory map in prism-exposed owls
  • Effects of Experience on Topographic Sensory Cortex Maps
    • Neural activity prior to sensory input plays a role in development – ferret visual development disrupted by interference with neuronal activity prior to eye opening
    • Early music training influences the organization of human auditory cortex – f MRI studies
  • Neuroplasticity in Adults
    • Mature brain changes and adapts
    • Neurogenesis (growth of new neurons) seen in olfactory bulbs and hippocampi of adult mammals
    • Researchers still looking to see if there is neurogenesis in other adult brain structures
  • Effects of Experience on the Reorganization of the Adult Cortex
    • Tinnitus (ringing in the ears) – produces major reorganization of 1° auditory cortex
    • Adult musicians who play instruments fingered by hand have an enlarged representation of the hand in right somatosensory cortex
    • Skill training leads to reorganization of motor cortex
  • Autism
    • 4 of every 10,000 individuals – 3 core symptoms:
      • Reduced ability to interpret emotions and intentions
      • Reduced capacity for social interaction
      • Preoccupation with a single subject or activity
    • Intensive behavioral therapy may improve function
    • Heterogenous – level of brain damage and dysfunction varies
  • Autism
    • Most have some abilities preserved – rote memory, ability to complete jigsaw puzzles, musical ability, artistic ability
    • Savants – intellectually handicapped individuals who display specific cognitive or artistic abilities
    • ~1/10 autistic individuals display savant abilities
    • Perhaps a consequence of compensatory functional improvement in the right hemisphere following damage to the left
  • Neural Basis of Autism
    • Genetic basis
      • Siblings of the autistic have a 5% chance of being autistic
      • 60% concordance rate for monozygotic twins
    • Several genes interacting with the environment
    • Brain damage tends to be widespread, but is most commonly seen in the cerebellum
  • Neural Basis for Autism
    • Thalidomide – given early in pregnancy – increases chance of autism
      • Indicates neurodevelopmental error occurs within 1 st few weeks of pregnancy when motor neurons of the cranial nerves are developing
      • Consistent with observed deficits in face, mouth, and eye control
    • Anomalies in ear structure indicate damage occurs between 20 and 24 days after conception
    • Evidence for a role of a gene on chromosome 7
  •  
  • Williams Syndrome
    • ~ 1 of every 20,000 births
    • Mental retardation and an uneven pattern of abilities and disabilities
    • Sociable, empathetic, and talkative – exhibit language skills, music skills and an enhanced ability to recognize faces
    • Profound impairments in spatial cognition
    • Usually have heart disorders associated with a mutation in a gene on chromosome 7 – the gene (and others) are absent in 95% of those with Williams
  • Williams Syndrome
    • Variety of abilities – like autistics
    • Evidence for a role of chromosome 7 – as in autism
    • Underdeveloped occipital and parietal cortex, normal frontal and temporal
    • “ elfin” appearance – short, small upturned noses, oval ears, broad mouths
  • Think About It
    • Compare and contrast autism and Williams syndrome
    • What do these disorders demonstrated about neurodevelopment?
    • How are such developmental disorders studied?