Development and Plasticity of the Brain Development o Very early embryo Primary Germ layers are formed in their locations during gastrulation Endoderm the most internal germ layer forms the lining of Mesoderm middle germ layer that forms muscle skeletal and the gut circulatory system Ectoderm outer layer A patch of tissue folds in to become the neural plate This fold pinches off Chemical signals triggered by genes tell the cells how to develop The tube will eventually become the brain o Human central nervous system forms when embryo is two weeks old Dorsal surface thickens forming a neural tube surrounding The forward end enlarges and differentiates into the hindbrain fluid filled cavity midbrain forebrain The rest of the neural tube becomes the spinal cord The fluid filled cavity becomes the central canal of the spinal cord and the four ventricles of the brain The fluid is the cerebrospinal fluid Development is triggered by neural inducer genes Two main neural inducers Cordin and Noggin Sonic Hedgehog major development gene because fruit flied with mutations to the SHH gene have extra bristles Stages of Development The development of neurons in the brain involves the following processes o Proliferation the production of new cells in the brain occurring early Some cells become stem cells continuing to divide Early in development the cells lining the ventricles divide Other cells become neurons or glia that migrate All neurons arise through proliferation Neurons are locked in and cannot divide o Migration the movement of the new neurons and glia to their in life locations Some don t reach destination until adulthood Occurs in a variety of directions throughout brain Occurs via cells following chemical paths in the brain immunoglobins and chemokines o Differentiation the forming of the axon and dendrite that gives the neuron its shape reached its target The axon grows first either during migration or once it has Once neuron reaches its destination dendrites form o Myelination glia produces the fatty sheath that covers the axons of some neurons Myelin speeds up transmission of neural impulses First occurs in the Spinal Cord and then in the hindbrain midbrain and forebrain Occurs gradually for decades Newborns have little Myelination in the brain o Synaptogenesis final stage of neural development and refers to the formation of the synapses between neurons Occurs throughout life Slows later in the lifetime Neurogenesis o Originally believed no new neurons were formed after early development o Research suggests otherwise Stem cells found in the interior of the brain generate daughter cells that can transform glia or neurons New olfactory receptors replace dying ones 90 days o Development of new neurons occur in other brain regions Ex Songbirds has replacement of neurons in singing area of brain o Stem cells differentiate into new neurons in the adult hippocampus of mammals and facilitate learning o However few neurons arise in adulthood in the Cortex o Axons must travel distances across brain to form connections o Sperry s 1954 research indicated that axons follow a chemical trail to reach their target Growing axons reach their target area by following a gradient of chemicals which they are attracted by some chemicals In amphibian Tectum the protein TOP dv is more concentrated target 1 cut optic nerve grows back to Tectum 2 cut nerve and rotate eye grows back so frog sees upside Axon Targeting The Suicide Cell down o Axons form synapses with cells in a location o Postsynaptic cells strengthen connections with others or eliminate connections with others o Formation of elimination of connections depends on input from incoming axons o If a neuron doesn t get a signal to stay it dies through Apoptosis o Neurotrophis are chemicals that promote survival of axons Nerve Growth Factor NGF or Brian Derived Neurotrophic factor BDNF The Vulnerable Developing Brain o Mutation o Malnutrition Thyroid deficiency due to low iodine impaired proliferation Low glucose impaired development o Infection o Toxins Fever impairs neuron proliferation Cocaine antidepressants cigs increase risk of ADD Fetal Alcohol Syndrome Hyperactivity Mental disability Motor problems Heart defects Alcohol suppresses glutamate and enhances GABA Developing neurons receive less signals Facial abnormalities Differentiation in Cortex components o Neurons in different parts of the brain differ in shape and chemical o Immature neurons transplanted from one part of the developing cortex to another develop the properties of the new location o Neurons transplanted at a later stage develop new properties but retain some old properties Ex ferret experiment The right hemisphere is trained to responds to a red light by turning to the light The rewired left hemisphere is tested and the ferret Dendrite Branching and Environmental Enrichment turns to the right o Brain has limited ability to reorganize itself in response to experience Axons and dendrites modify their structure throughout lifetime to learning Dendrites grow new spines o The gain and loss of spines indicated new connections which relates Rats raised in enriched environment develop a thicker cortex and increased dendritic branching o The thickness of the cerebral cortex declines in old age but less in those physically active Experience o People who learned to read show more gray matter cell bodies and greater thickness in part of the corpus Callosum o Studies reveal Temporal love of musicians in the right hemisphere is 30 larger than non musicians Thicker gray matter in part of brain responsible for hand control and vision of professional keyboard players Focal Hand Dystonia o Condition caused by extensive reorganization of sensory thalamus and cortex so that touch responses to one finger overlap those of another Adolescents o More impulsive than adults leading to drinking risky driving sex etc o Antisaccade task looking away from a powerful attention getter Improves during teen years o Show stronger reward responses o Weaker prefrontal cortexes response when should inhibit behavior may cause impulsivity Old Age o Neurons alter synapses slower o Brain loses volume o Research underestimates older people People vary in respect to intellectual decline Have greater knowledge and experience Plasticity o The ability of the brain to change o Organization and location of brain processing can change due to learning or injury o Decreases with age o The greatest plasticity is during infancy
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