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5. Development and Plasticity of the Brain.pdf

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PSYC 2240
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5 Development and Plasticity of the BrainWednesday June 26 2013849 PMMain IdeasNeurons begin by migrating to their proper locations and developing axons which extend to their correct targets by followingchemical 1pathwaysThe nervous system at first forms far more neurons than it needs and then eliminates those that do not establish suitable connections or 2receive sufficient input It also forms excess synapses and discards the less active onesExperiences especially early in life alter brain anatomy3Many mechanisms contribute to recovery from brain damage including restoration of undamaged neurons to full activity regrowth of 4axons readjustment of surviving synapses and behavioral adjustments51 Development of the BrainMaturation of the Vertebrate BrainThe CNS begins to form when the embryo is about 2 weeks oldThe brain and spinal cord begin as folding lips surrounding a fluidfilled canalGrowth and Development of Neurons5 processes in the development of neuronsProliferationthe production of new cells1Migrationthe movement of neurons towards their eventual destinations in the brain after cells have differentiated as neurons 2or gliaDifferentiationformation of the axon and dendrites that gives a neuron its distinctive shape3Myelinationthe process by which glia produce the insulating fatty sheaths that accelerate transmission in many vertebrate 4axonsSynaptogenesisthe formation of synapses5New Neurons Later in LifeResearchers used to believe that new neurons were not developed after early infancy but exceptions have been foundStem cellsin the nose remain immature and replace dying olfactory receptors which have a halflife of only 90 daysStem cells differentiate into new neurons in the adult hippocampusof mammals which facilitate memoryPathfinding by AxonsChemical Pathfinding by AxonsSpecificity of Axon ConnectionsSensory axons are able to find their way to their correct targets by following a chemical trailChemical GradientsA Growing axon follows a path of cellsurface molecules attracted by some chemicals and repelled by others in a process that steers the axon in the correct directionCompetition Among Axons as a General PrincipleWhen axons initially reach their targets each one forms synapses onto many cells in approximately the correct location and each target receives synapses from many axonsAt first axons make trial connections with many postsynaptic cells and then each postsynaptic cell strengthens some synapses and eliminates othersThe brain works to wire itself Carla J ShatzNeural Darwinismwe start with more neurons and synapses than we keep and only the most successful ones surviveDeterminants of Neuronal SurvivalMuscles do not determine how many axons form they determine how many survive discovery by Rita LeviMontalciniThe sympathetic nervous system forms far more neurons than it needs When one of its neurons forms a synapse onto a muscle that muscle delivers a protein called nerve growth factor NGFthat promotes the survival and growth of the axon An axon that does not receive NGF degenerates and the cell body diesIe if a neurons axon does not make contact with an appropriate postsynaptic cell by a certain age the neuron kills itself through a process called apoptosisa programmed mechanism of cell deathNGF cancels apoptosisNGF is a neurotrophina chemical that promotes the survival and activity of neuronsOther neurotrophins include brainderived neurotrophic factor BDNF etcTo prevent apoptosis a neuron must receive neurotrophins from the incoming axons as well as from its own axons target cellEach brain area has a period of massive cell death which indeed can indicate development and maturation in that area Textbook Notes Page 1
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