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