1. Brain Development & Neuroplasticity

2. Neurodevelopment an ongoing process; the nervous system is plastic
A complex process
Genetics order but experience modifies
Dire consequences when something goes wrong

4. 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!

5. 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

6. 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)

7. 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

8. 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

9. How the neural plate develops into the neural tube during the third and fourth weeks of human embryological development

10. 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

11. 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

12. Somal translocation and glia-mediated migration
Somal translocation and glia-mediated migration

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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”
As we age, old connections are deleted “synaptic prunining”

20. Neurons must have purpose to survive
Weak or ineffective connections “pruned”
Plasticityenables process of developing and pruning connections allowing the brain to adapt to itself to the environment

21. 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

22. Plastiticy cont. Age-dependent factor Occurs under two conditions
Young dev brains more plastic
Occurs under two conditions
Normal development
In response to damage or disease

23. Hemispherectomy Removal of one hemisphere
Last effort to control seizures

24. 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

25. 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