PSB2000 Exam 2 Lecture Summary C Robison Instructor Lecture 10 Brain Development and Plasticity Over the course of development the brain transforms from a single undifferentiated cell into billions of highly interconnected neurons This happens in a progression of stages using various genetic and molecular markers to trigger development Even after development mechanisms of plasticity remain that help the brain learn remember and adapt to changes Anatomical Development Anatomical development begins during embryogenesis when the embryo is forming and continues into early adulthood This development uses molecular signals that will be covered later Brain tissue comes from the ectoderm the outer layer of tissue in the developing embryo First a groove forms The groove folds up into a neural tube which is brought into the developing embryo Different areas of the brain start to develop from the tube The three embryonic brain areas are the prosencephalon mesencephalon and rhombencephalon The retina of the eye is part of the prosencephalon The development of these different areas is triggered by neural inducer genes These cause different types of growth in different areas of the brain These inducers express signaling proteins such as cordin noggin and SHH sonic hedgehog At birth the infant brain is roughly of the size of an adult brain It nearly triples in size in the first year of life The infant s brain is unmyelinated no white matter White matter continues to grow into adulthood The prefrontal cortex is the last area of the brain to mature This can take upwards of 20 years to happen Development Mechanisms There are six major processes in play during development Some of these last into adulthood while others are largely restricted to embryogenesis or childhood Proliferation stem cells start dividing during embryogenesis All neurons are formed by proliferation since they cannot divide once mature Migration newly formed neurons travel up the spokes of radial glia cells They start at the subventricular zone and move toward the outside of the brain Differentiation there are many different types of neurons Differentiation is when neurons decide what type of cell to become based on many factors such as what chemicals are being released by nearby cells Myelination myelin forms around the axons of some neurons All neurons start out ummyelinated Without myelin there could not be effective long range communication within the brain Synaptogenesis the formation of new synapses Axons follow chemical signals neurotrophins out to locations where they can synapse Page 1 of 16 PSB2000 Exam 2 Lecture Summary C Robison Instructor Pruning neurons that don t form enough successful synapses wither and may die This is called pruning Most of your brain cells never make it because they re unsuccessful and get pruned Plasticity Plasticity is the brain s ability to change This helps fine tune our brain s systems during development and is important for recovery after brain injury The change in synapse structure as a result of learning and experience is a form of plasticity Plasticity is greatest early in development Having a complex interactive enriched environment during development leads to more complex neurons Underused areas of the cortex can be invaded by nearby areas This is why deaf or blind individuals often have sound and vision areas of the cortex activated by different things Plasticity often involves the growth or degeneration of axons both are important Plasticity is often caused by the activation of cells such as by a puff of glutamate If a nerve is damaged nearby nerves can take its place by following neurotrophic factors to the cell that the damaged neuron used to synapse with Page 2 of 16 PSB2000 Exam 2 Lecture Summary C Robison Instructor Lecture 11 Brain Damage Neurogenesis Damage to the brain can result in a profound impairment in function Some of this function can be recovered through mechanisms of plasticity Neurogenesis the growth of new neurons in adults is important in several aspects of behavior Brain Damage Physical trauma to the skull or brain can result in brain damage such as during a car accident Additionally strokes are a major cause of brain damage This is when a blood clot blocks flow to part of the brain ischemia or when a blood vessel bursts in the brain resulting in a brain bleed hemorrhage Bleeding in the brain kills cells in several ways o Lack of oxygenated blood starves cells o Pressure in the head can squish the cells to death o The iron in the blood is toxic to neurons and can kill them o Dying cells can cause nearby cells to die The brain areas adjacent to the area of damage are called the penumbra Dying cells release cell death factors and also fire erratically Rapid firing can fry cells in the penumbra Death factors can trigger apoptosis Cells in the penumbra can be saved by restoring blood flow or cooling the brain Neuroprotective drugs can also help This can block apoptosis Apoptosis is the orderly death of a cell as opposed to necrosis It is often triggered by cell death signals or by overexcitation called excitotoxicity The cell shrinks and sends off parts of itself for recycling It sends signals for migroglia to eat the remaining cell Apoptosis is caused by a chemical cascade in the cell If the cascade is stopped early enough the cell can be saved Once the cell reaches a point of no return it will die Recovery From Damage There is often some degree of recovery after an individual suffers from brain damage This is due to the various mechanisms of plasticity present in the brain Young people have more plasticity than the elderly but plasticity is present to some extent in all individuals When a brain area is damaged axons to that area retract They may rewire to other brain areas by following neurotrophic signals They branch off to find functioning neurons Cells that don t find a new neuron to contact may die This is called diaschisis when a brain area degenerates due to damage in a distant structure that normally activates that area New axons grow when filopodia growth cones follow molecular signals to form a synapse Sometimes they follow the wrong signal and re wire things a bit wrong Phantom limb syndrome is a symptom of plasticity after the brain has stopped receiving signals from a part of the body that s been removed Neurogenesis Neurogenesis is the growth of new neurons after embryogenesis It used to be thought that no Page 3 of 16 PSB2000 Exam 2 Lecture
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