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Copyright © 2007 by Allyn and Bacon
Chapter 7Development of the Nervous System
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Copyright © 2007 by Allyn and Bacon
Neurodevelopment
Neural development – an ongoing process, the nervous system is plastic
Complex Experience plays a key role Dire consequences when something
goes wrong
Copyright © 2007 by Allyn and Bacon
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?
Copyright © 2007 by Allyn and Bacon
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?
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Phases of Development
Ovum + sperm = zygote Cells then multiply and
DifferentiateMove and take their appropriate positionsMake the needed functional relations with
other cells Developing neurons accomplish these
things in 5 phases
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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
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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
Copyright © 2007 by Allyn and Bacon
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)
Copyright © 2007 by Allyn and Bacon
Copyright © 2007 by Allyn and Bacon
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
Copyright © 2007 by Allyn and Bacon
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
Copyright © 2007 by Allyn and Bacon
Migration
Two types of neural tube migrationRadial migration – moving out – usually by
moving along radial glial cellsTangential migration – moving up
Two methods of migrationSomal – an extension develops that leads
migration, cell body followsGlial-mediated migration – cell moves along a
radial glial network
Copyright © 2007 by Allyn and Bacon
Copyright © 2007 by Allyn and Bacon
Copyright © 2007 by Allyn and Bacon
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
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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
Copyright © 2007 by Allyn and Bacon
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
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Axon growth
Pioneer growth cones – the 1st 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
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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
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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 deathDestroy some cells > increased survival of
remaining cells Increase number of innervating axons >
decreased proportion survive
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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”
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Postnatal Cerebral Development in Human Infants Postnatal growth is a consequence of
SynaptogenesisMyelination – 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
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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
Copyright © 2007 by Allyn and Bacon
Copyright © 2007 by Allyn and Bacon
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?
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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 corticesgreater dendritic developmentmore synapses per neuron
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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
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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
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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 – fMRI studies
Copyright © 2007 by Allyn and Bacon
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
Copyright © 2007 by Allyn and Bacon
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
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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
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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
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Neural Basis of Autism
Genetic basisSiblings of the autistic have a 5% chance of
being autistic60% 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
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Neural Basis for Autism
Thalidomide – given early in pregnancy – increases chance of autism Indicates neurodevelopmental error occurs within 1st
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
Copyright © 2007 by Allyn and Bacon
Copyright © 2007 by Allyn and Bacon
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
Copyright © 2007 by Allyn and Bacon
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
Copyright © 2007 by Allyn and Bacon
Think About It
Compare and contrast autism and Williams syndrome
What do these disorders demonstrated about neurodevelopment?
How are such developmental disorders studied?