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Chapter 9

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University of Guelph
PSYC 2410
Boyer Winters

Chapter 9: Development of the Nervous System - From Fertilized Egg to You The brain is a plastic (changeable) living organ that continuously changes in response to its genetic program and environment. Neurodevelopment = neural development • Early experiences play a critical role in the development of the human cerebral • The Case of Genie: Limited movement and ability to learn and grow properly as an infant resulted in unfixable abnormal behaviour for the rest of her life. 9.1 Phases of Neurodevelopment − Begins as a zygote (fertilized egg) which divides into two daughter cells and so forth − Cells must differentiate into specific types of cells, migrate and properly align in specific spots and establish appropriate functional relations with other cells − this is accomplished by 5 phases: (1) induction of the neural plate, (2) neural proliferation, (3) migration and aggregation, (4) axon growth and synapse formation, and (5) neuron death and synapse rearrangement Induction of the Neural Plate − Neural Plate: small patch of ectodermal tissue on the dorsal surface of the developing embryo that appears 3 weeks after conception − Development is induced by chemical signals from the mesodermal layer (organizer) − The earliest cells of the human embryo are totipotent (able to develop into any cell type). When the neural plate develops, some cells become multipotent (able to develop into most types in the mature nervous system, but not other kinds of cells) − Neural plate cells are embryonic stem cells - (1) unlimited capacity for self- renewal, (2) ability to develop into different types of mature cells - some cells lose their ability to become any cell as the neural plate forms the neural tube − Unlimited renewal is a result of one daughter cell becoming a body cell and the other a stem cell following division, however, eventually errors accumulate during mitosis and the process is disrupted − The neural plate folds to form the neural grove, where the groove's lips fuse forming the neural tube where the inside eventually becomes the cerebral ventricles and the spinal canal. By 40 days following conception, 3 swellings are visible indicating the forebrain, midbrain and hindbrain Neural Proliferation − Beginning once the neural tube is formed, the cells of the tube increase greatly in number − Does not occur equally nor simultaneously - most division occurs in the ventricular zone (region adjacent to the ventricle; Ventricle - fluid-filled center of the tube) − Proliferation pattern mediated from two organizer areas of the tube - the floor plate (along the midline of the anterior surface) and the roof plate (along the midline of the dorsal surface) Migration and Aggregation − Cells of immature form since they lack processes of mature neurons, migrate to appropriate locations − Time and location govern migration; in a given region of the tube, neuron subtypes arise on precise and predictable schedule and then migrate together to prescribed location − Cells migrate in two ways: (1) Radial migration: from the ventricular zone in a straight line outward toward the outer wall of the tube, or (2) Tangential migration: at a right angle to radial migration (parallel to the tube's wall) - most cells engage in both − Two methods by which cells migrate: (1) Somal translocation: an extension grows from the developing cell in the general direction of the migration where the cell body moves along the process and it trailing processes retract, or (2) Glia-mediated migration: walls of the tube thicken with temporary network of glial cells (radial glial cells) in which most cells engage in radial migration along this network − Cortical development occurs in an inside-out pattern where each wave of cortical cells migrate through already formed lower layers of the cortex to reach its destination in radial pattern − Neural Crest: dorsal to the tube, formed from cells that break off from the tube as it is being formed − Chemicals guide migrating neuron by attracting or repelling them - some of these chemicals are released by glial cells − Aggregation is when these now migrating neurons align themselves appropriately − Migration and aggregation mediated via cell-adhesion molecules (CAMs) located on surfaces of neurons and other cells - they are believed to be factors in some neurological disorders − Gap junctions also play a role where the connexins (tubes) allow for exchange of cytoplasm Axon Growth and Synapse Formation − Growth cone: amoebalike structure at the end of each growing tip of an axon or dendrite, which extends and retracts filopodia extensions to find the correct route − Roger Sperry: cut optic nerves in frogs, rotated their eyeballs, and waited for retinal ganglion cell axons to regenerate (only occurs in frogs). Provided evidence that each retinal ganglion cell grew back to the same point of the optic tectum (mammals - superior colliculus). Thus produced the chemoaffinity hypothesis of axon development: each postsynaptic surface in the NS releases a specific chemical where each growing axon is attracted to as a target during neural development and regeneration − Revised hypothesis: growth cones are influenced by a series of chemical signals along the pathway, not just one − Pioneer growth cones: first growth cone to travel along a particular route in a developing NS - subsequent growth ones follow − Fasciculation: the tendency of developing axons to grow along the paths established by preceding axons − Axonal development is in a topical array to minimize the volume of neural connections. Topographic gradient hypothesis: axons growing from one topographic surface to another are guided to specific targets that are arranged on the terminal surface in the same way that they're arranged on the original surface − The ephrin family is strongest in guiding molecules − Synaptogenesis is the formation of new synapses which deends on the presence of glial cells, namely astrocytes; Developing neurons need high levels of cholesterol during synapse formation, provided by astrocytes. They also process, transfer, and store information supplied by neurons Neuron Death and Synapse Rearrangement − About 50% more neurons than required are produced, thus cell death is an active process − Necrosis: passive cell death; Apoptosis: active cell death - Necrotic cells spill their contents into extracellular fluid potentially causing inflammation whereas apoptotic cells cleave internal structures and package them in membranes (which attract scavenger microglia) prior to breaking apart. However, blocking apoptosis can result in cancer − Two triggers of apoptosis: (1)pre-programmed for early death after they've fulfilled their function, and (2) failing to obtain life-preserving chemicals supplied by their targets − Neurotrophins (i.e. nerve growth factor - NGF) are the most prominent class of life-preserving chemicals, and function as axon guidance molecules, as well as stimulating synaptogenesis − Cell death results in a massive synapse rearrangement which focuses on the output of each neuron on a smaller number of postsynaptic cells thus increasing the selectivity of transmission 9.2 Postnatal Cerebral Development in Human Infants The human brain develops more slowly than other species, thus not achieving full maturity till late adolescence. The prefrontal cortex is the last part of the human brain to reach full maturity. Postnatal Growth of the Human Brain − Human brain volume quadruples between birth and adulthood − The olfactory bulb and the hippocampus are the only two structures where many new neurons continue to be created during adult years - the rest are developed by 7 months of prenatal development − Postnatal growth is a result from (1) synaptogenesis - general increase within the cortex shortly after birth, (2) myelination of axons - to increase axonal connection speed; sensory followed by motor, prefrontal continues in adulthood, and (3) increased branching of dendrites - duplicating the original pattern of neural migration − Regressive changes as well as growth; synaptic loss once maximal synaptic density achieved (note: overproduction of synapses may underlie greater plasticity in a younger brain) − Cortical white matter grows slowly and steadily until early adulthood; cortical rgay matter growth follows an inverted U pattern, correlated with functional maturity (sensory and motor reach maturity before cognitive areas) Development of the Prefrontal Cortex − Prefrontal cortex regions play roles in (1) working memory - relevant information accessible for short periods of time while task being completed, (2) planning and carrying out sequences of actions, (
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