Brain Growth and Neural Development Reflexes

Dynamic Action System Small changes during development lead to major changes in structure and functioning later on Prenatal and postnatal Changes attributed to “plasticity”  What does this mean?  Example: ocular dominance 7 Primary Processes in Neural Dev

Neurogenesis (step 1) Proliferation of cells/prenatal Neurons produced in neural tube during prenatal development Glial cell production starts before birth- continues beyond birth  forms a network that the neurons use to migrate  Increases efficiency

Migration (2) Cells move through passive displacement (pushed out by newly forming cells) Active displacement- along the glial cell structures Plasticity- the degree to which a developing structure or behavior is modifiable due to experience.  Example: cell destined for visual area misses mark ends up in auditory area differentiates Migration Failure

Axon Growth (3) Axons grow toward specific targets Guidance occurs as a result of neurotrophic factors-  family of proteins - growth and survival of the neuron  Chemical attraction or repulsion causes growth cone of axon to change direction

Maturation (4) Neurons are formed and migrating before birth (by end of 2nd trimester) Neurons are functional but not optimally (at adult levels)- we see inconsistent firing patterns Activity pattern changes with experience to become more efficient and consistent See Kaylin video

Synaptogenesis (5) Increase in synaptic connections Dendritic branching begins increasing shortly before birth These high levels are associated with infant relflexes E.g. visual synapses of adult birth 8 months, synapses are at max levels years, adult levels (50-60% of max level)

Decrease in # of synapses (6) Big increase, then gradually decrease in number after birth (~8 mo) Apoptosis- programmed cell death- predetermined lifespan of the cell related to neurotrophic factors

Decrease in # of Synapses (6) -Pruning Pruning- increased activity in specific areas associated with a decrease of the neuronal network.  decrease in density occurs at the same time as regression of infantile reflexes acquisition of specific abilities  seems to be a critical mass of synapses required for onset of specific skills acquisition. as skills are refined  pruning

Myelination (7) Responsible for increased wt & size of brain  Neurons in place after birth- axons become coated with fatty substance after birth (Glial Cells).  Glial cells Taste, smell, touch are myelinated partially or fully at birth vision- process complete sometime around 5-8 months language processing pathways begin around 12 months rapid expansion of myelin sheath around months- Result? myelination associated with acquisition of new skills- speed of transmission brain growth spurt- last trimester to end of 2nd year Additional growth spurt 5-8 years

MRI Studies

Brain Growth Size  Functionally developed at birth birth brain weighs ¾ - 1 lb (25% of adult size) 6 months, 50% of adult size (1.5-2 lbs) 2 years 75% of adult size ( lbs) Fetal Brain Growth

Why such growth? What accounts for increased size?  Addition of glial cells  Myelination of the axons Shape As size increases- shape changes Deeper folds More convolutions Increases the surface area