Brain development

3 basic processes: 1. Produce neurons  2. Place neurons where they belong   3. Connect neurons

• Neural tube will develop into CNS. Early in development, main division of CNS.
• Appear as distinct sections. These will form ventricles (cortico-spinal-fluid-filled chambers of brain) - shock absorber in the NS. Forebrain, optic vesicles develop into retina & optic nerve
• Production and placement: Neurons+glia develop from neural stem cells at ventricle surface

Step 1 - cell proliferation: Divison a. Undifferentiated stem cell grows an extension to surface. b. Cell nucleus moves up+down and duplicates DNA. c. Nucleus moves down, cell retracts extension and divides into two.  d. Cells behave differently depending on whether stem cell divided horizontally or vertically.

• Vertical split = both daughter cells repeat process
• Horizontal split = One daughter cell repeats process, other moves away and won't divide again
• Cortex = many different types of neurons organised in structured layers.

Step 2 - cell migration: Different cells have different places of origin. Pyranmidal cells/astrocytes originate from dorsal areas and migrate vertically. Inhibitory interneurons + oligodendroglia originate from ventral areas and migrate laterally.

Pathway of developing, undifferentiated neurons: Radial glia cells in ventricular zone extend thin processes towards surface in ordered pattern (ventricular zone is a proto-map of cortex). Developing neurons crawl along the process towards their destination. Radial glia cells in ventricular zone extend thin processes towards the surface.

Step 3 - connecting cells: 3 problems to be solved...1. Find right direction 2. Find right target structure  3. Find right target cells inside target structure

Pathways of undifferentiated neurons: a. Cells to migrate take up residence in subplate layer. b. Cells migrate to cortical plate. c. 1st to layer VI, then V, IV e.t.c (backwards). d.Interneurons move in sideways.

• Problems: 1. Each hemisphere receives signals from and sends commands to contralateral. Right half of visual field represented by left visual cortex.
• 2. Signals guide acon development, only attracted to specific target structure chemicals. Growing axons guided by chemical signals in environment. Different chhemical affinities for different neuron locations, affinities can be modified by chemical signals in environment.
• 3. How can chemical cues differentiate between cells? Input&output must be linked, unlikely to be achieved by chemical signals alone. Wiring of NS depends on dynamic, coordinated, electrical activity.

Use it or lose it: Axons synapse with cells in target structure indiscriminately (initially too many synapses develop)

• Pruning = Only useful synapses maintained - useless ones disappear
• Useful = will link 2 neurons which mostly are either BOTH active or both silent
• Useless = links 2 neurons which are RARELY both active/silent at the same time

Useful synapses are useful because...

• •#1. Creates ordered structure which represents structure inherent in environment
• •#2. Otherwise, energy is wasted on something that contributes nothing
• •#3. Prenatally, correlated activity is established through spontaneous waves or retinal activity

Coordinating eyes: Visual input is needed to coordinate both eyes. Newborn babies have to learn to see. If activity of pre and postsynaptic cell is uncorrelated, synapses between these 2 cells disappear (out of sync, lose their link)