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Chapter 9

PSYC370- Chapter 9.docx

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Queen's University
PSYC 370
Monica Valsangkar- Smyth

Chapter 9: Development of the Nervous System- From Fertilized Egg to You - The brain is plastic, living organ that continuously changes in response to its genetic programs and environment - Neurodevelopment starts with the fertilized egg cell and ends with a functional adult brain - Emphasis on: 1. Amazing nature of neurodevelopment 2. Important role of experience in neurodevelopment 3. Dire consequences of neurodevelopmental errors - Ovum plus sperm = zygote - During cell division, cells must differentiate (become muscle cells, multipolar neurons, glial cells), make their way to appropriate sites and align themselves with the cells around them to form particular structures, and must establish appropriate functional relations with other cells - Developing neurons accomplish these things in five phases: 1. Induction of neural plate 2. Neural proliferation 3. Migration and aggregation 4. Axon growth and synapse formation 5. Neuron death and synapse rearrangement - Neural plate: a small patch of ectodermal tissue on the dorsal surface of the vertebrate embryo, from which the neural groove, the neural tube, and, ultimately, the mature nervous system develop o Becomes recognizable three weeks after conception - Ectoderm is the outermost of the three layers of embryonic cells: ectoderm, mesoderm, and endoderm - Mesoderm layer: the middle of the three layers in the developing embryo o Referred to as the organizer; sends chemical signals that induce the development of the neural plate - Tissue taken from the dorsal mesoderm of one embryo and implanted beneath the ventral ectoderm of another embryo induces the development of an extra neural plate on the ventral surface of the host - Totipotent: capable of developing into any type of mature body cell (earliest cells of embryo) - As embryo develops cells become more specified - Each cell of the neural plate has the potential to develop into most types of mature nervous system cell, but not others - Multipotent: capable of developing into a limited number of types of mature body cell - Stem cells: developing cells that have the capacity for self-renewal and the potential to develop into various types of mature cells (cells of neural plate are embryonic stem cells) - As neural plate develops into neural tube the cells become more specified o They are still capable of unlimited self-renewal and are still multipotent; they are termed glial stem cells and neural stem cells - When stem cells divide, two daughter cells are created: one that becomes a body cell, and one that develops into another stem cell - Errors that accumulate can disrupt the process; reason why stem cultures do not last forever - Neural plate folds to form neural groove and the neural tube - Insides of the neural tube eventually becomes the cerebral ventricles and spinal canal - 40 days after conception three swellings become visible which will become the forebrain, midbrain and hindbrain - Neural proliferation: the rapid increase in the number of neurons that follows the formation of the neural tube o Does not occur simultaneously or equally in all parts of the tube - Ventricular zone: the region adjacent to the ventricle in the developing neural tube; the zone where neural proliferation occurs - In each species, the cells in different parts of the neural tube proliferate in a particular sequence that is responsible for the pattern of swelling and folding that gives the brain of each member of that species the characteristic shape - Complex pattern of proliferation is in part controlled by chemical signals from two organizer areas in the neural tube: floor plate (runs along the midline of the anterior surface of the tube) and roof plate (runs along the midline of the dorsal surface of the tube) - Migration: the movement of cells from their site of creation in the ventricular zone of the neural tube to their ultimate location in the mature nervous system o Cells are still in an immature form, lacking the processes (i.e., axons and dendrites) that characterize mature neurons - Time and location govern migration in the developing neural tube - In given regions of the tube subtypes of neurons arise on a precise and predictable schedule and then migrate together to their prescribed destinations - Radial migration: movement of cells in the developing neural tube from the ventricular zone in a straight line outward the tube’s outer wall - Tangential migration: movement of cells in the developing neural tube in a direction parallel to the tube’s walls - Most cells engage in both migrations - Two methods by which developing cells migrate o Somal translocation: one of two major modes of neural migration, in which an extension grows out from the undeveloped neuron and draws the cell body up into it o Glial-mediated migration: one of two major modes of neural migration during development, by which immature neurons move out from the central canal along radial glial cells - Radial glial cells: glial cells that exist in the neural tube only during the period of neural migration and that form a network along which radial migration occurs - Inside-out pattern: the pattern of cortical development in which orderly waves of tangential migrations progress systematically from deeper to more superficial layers - Many cortical cells engage in long tangential migrations to reach their final destinations; patterns of proliferation and migration are different for different areas of the cortex o Migration of interneurons is particularly complex - Neural crest: the structure that is formed by cells breaking off from the neural groove during the formation of the neural tube and that develops into the peripheral nervous system - Numerous chemicals guide migrating neurons by attracting or repelling them - Aggregation: the alignment of cells within different areas of the embryo during development to form various structures - Cell-adhesion molecules (CAMs): molecules on the surface of cells that have the ability to recognize specific molecules on the surface of other cells and bind to them - Elimination of just one type of CAM in a knockout mouse has been shown to have a devastating effect on brain development - Gap junctions (between neurons) are bridged by narrow tubes called connexions through which cells can exchange cytoplasm - Once neurons reach their location axons and dendrites begin to grow from them o Projections must grow to appropriate targets for nervous system to function - Growth cone: amoebalike structures at the tip of each growing axon or dendrite that guides growth to the appropriate target - Roger Sperry’s axon regeneration experiments with frogs (cutting the optic nerve and seeing how it regenerates) - Chemoaffinity hypothesis: the hypothesis that growing axons are attracted to the correct targets by different chemicals released by the target sites - Several guidance molecules for axon growth have been identified - Growth cones seem to be influenced by a series of chemical signals along the route - Pioneer growth cones: the first growth cones to travel along a particular route in a developing nervous system - Subsequent growth cones embarking on the same journey follow the routes blazed by the pioneers - Fasciculation: tendency of developing axons to grow along the paths established by preceding axons - It has been suggested that topographic maps evolved as a means of minimizing the volume of neural connections in the brain - Evolutionary pressure for richly connected neurons to be as close as possible to each other - Topographic gradient hypothesis: the hypothesis that axonal growth is guided by the relative position of the cell bodies on intersecting gradients, rather than by point-to-point coding of neural connections - A single neuron can grow an axon on its own but it takes coordinated activity in at least two neurons to create a synapse
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