Brain Development & Neuroplasticity. Neurodevelopment an ongoing process; the nervous system is...
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Transcript of Brain Development & Neuroplasticity. Neurodevelopment an ongoing process; the nervous system is...
Brain Development & Neuroplasticity
Neurodevelopmentan ongoing process; the nervous system is plastic
A complex process Genetics order but experience modifies Dire consequences when something goes
wrong
Brain Development During dev: 250,000 neurons per minute At birth..100 billion neurons; 50 trillion to 1
quadrillion synapses Use it or lose it!
Phases of Development
Developing neurons accomplish these things in five phases Induction of the neural plate Neural proliferation Migration and aggregation Axon growth and synapse formation Neuron death and synapse rearrangement
Induction of the Neural Plate A patch of tissue on the dorsal surface of the
embryo becomes the neural plate Visible three weeks after conception Three layers of embryonic cells
Ectoderm (outermost) Mesoderm (middle) Endoderm (innermost)
Neural Tube Defects Neural tube closes about 28 days after
fertilization—WOW! Anencephaly-missing or partial dev of cerebral
hemispheres Spina bifida
Nearly 50-70% can be prevented with folic acid in diet
Induction of the Neural Plate (continued)
Neural plate cells are often referred to as embryonic stem cells
Have unlimited capacity for self renewal Can become any kind of mature cell
Totipotent – earliest cells have the ability to become any type of body cell
Multipotent – with development, neural plate cells are limited to becoming one of the range of mature nervous system cells
How the neural plate develops into
the neural tube during the third
and fourth weeks of human
embryological development
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:
three swellings at the anterior end in humans will become the forebrain, midbrain, and hindbrain
Proliferation is chemically guided by the organizer areas – the roof plate and the floor plate
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
Somal translocation and glia-mediated migration
http://www.youtube.com/watch?v=ZRF-gKZHINk
http://www.youtube.com/watch?v=4TwluFDtvvY&feature=related
Aggregation After migration, cells align themselves with
others cells and form structures Cell-adhesion molecules (CAMs)
Aid both migration and aggregation CAMs 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 filopodia as if finding its way
http://www.youtube.com/watch?v=n_9YTeEHp1E&NR=1
Axon Growth A series of chemical signals exist along
the way – attracting and repelling Such guidance molecules are often
released by glia Adjacent growing axons also provide
signals
Axon Growth (continued)
Pioneer growth cones – the first to travel a route, interact with guidance molecules
Fasciculation – the tendency of developing axons to grow along the paths established by preceding axons
Synaptogenesis Formation of new synapses (at birth each neuron
has ~2500 synapses; 2-3 years old 15,000) Depends on 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
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 The more targets, the fewer cell deaths Destroying some cells increases survival rate
of remaining cells Increasing number of innervating axons
decreases the proportion that survives
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” http://www.youtube.com/watch?v=gYWUTBM8tTo&feat
ure=related
As we age, old connections are deleted “synaptic prunining”
Neurons must have purpose to survive
Weak or ineffective connections “pruned” Plasticityenables process of developing
and pruning connections allowing the brain to adapt to itself to the environment
Plasticity brain changes and adapts Brain’s ability to reorganize itself by forming
new neural connections Allows neurons to compensate for injury or
disease Allows neurons to adjust to new activities /
change in environment
Plastiticy cont.
Age-dependent factor Young dev brains more plastic
Occurs under two conditions Normal development In response to damage or disease
Hemispherectomy Removal of one hemisphere Last effort to control seizures http://www.youtube.com/watch?v=TSu9HG
nlMV0
Neuroplastic Responses
Change in neurons Increaed no. of neurons (hippocampus) Increased dendritic branching Increased efficiency in NT production
Increased in no. of synapses between neurons
Neurogenesis Physical activity and environmental condition affect
proliferation and survival of neurons
Serotonin believed to play a key role in neurogenesis
In lobsters depletion of serotonin reduced neurogenesis in olfactory areas
Lab simulation
http://www.wellesley.edu/Biology/Concepts/Simulation/labsimulation.html