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Chapter 9 Textbook Notes.docx

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University of Guelph
PSYC 2410
Boyer Winters

NEUROSCIENCE – CHAPTER 9: DEVELOPMENT OF THE NERVOUS SYSTEM - The brain is a plastic (changeable) living organ that continuously changes in response to its genetic programs and environment 9.1 – Phases of Neurodevelopment - To start, there is a zygote (a single cell forms by the combination of an ovum and a sperm - Three things must occur for neurodevelopment (other than cell multiplication): o Cells must differentiate (some become muscle cells, some become glial cells, etc) o Cells must make their way to appropriate sites and align themselves with the cells around them to form particular structures o Cells must establish appropriate functional relations with other cells o Cells accomplish these things in 5 phases (induction of the neural plate, neural proliferation, migration and aggregation, axon growth and synapse formation, and neuron death and synapse rearrangement) - 1 Stage – Induction of the neural plate: o Neural plate: a small patch of ectodermal tissue on the dorsal surface of the developing embryo; this becomes the human nervous system eventually o Ectoderm is the outermost of the three layers (mesoderm and endoderm being the other ones) o Development of the neural plate is induced by chemical signals from the mesoderm layer (an area that is consequently referred to as an organizer) o The earliest cells of the embryo are totipotent (they have the ability to develop into any type of cell in the body if transplanted to the appropriate site) o As the embryo develops, the destiny of various cells becomes more specified (the cells lose their ability to become different kinds of cells) – these cells are called multipotent o Embryonic stem cells = cells of the neural plate o Stem cells = cells that self-renew if maintained in an appropriate cell culture; have the ability to develop into different types of mature cells  When a stem cell divides, one of the cells turns into a type of body cell; the other one turns into another kind of stem cell o The neural plate folds to form the “neural groove”; the lips of this groove fuse to form the “neural tube” o The inside of the neural tube eventually becomes the cerebral ventricles and spinal canal o 40 days into conception, three swellings are visible that will eventually turn into nd the forebrain, midbrain and hindbrain - 2 Stage – Neural proliferation o Once the neural tube has been formed from the joining of the lips of the neural groove, the cells of the tube begin to proliferate (increase greatly in number) o Most cell division in the neural tube occurs in the ventricular zone (the region adjacent to the ventricle – the fluid-filled center of the tube) o The floor plate (runs along the midline of the anterior surface of the tube) and the roof plate (runs along the midline of the dorsal surface of the tube) control proliferation - 3 Stage – Migration and aggregation: o Migration:  Once cells have been created through cell division in the ventricular zone, they migrate to the appropriate target region  Cells are still lacking the processes that characterize mature neurons at this stage  Two major factors govern migration: time and location (subtypes of neurons arise at a precise time and then migrate to their destinations)  Cell migration can be of two kinds  1. Radial migration: proceeds from the ventricular zone in a straight line outward toward the outer wall of the tube  2. Tangential migration: occurs at a right angle to radial migration (parallel to the tube’s walls)  Two methods by which developing cells migrate:  1. Somal translocation: an extension grows from the developing cell in the general direction of the migration  2. Glia-mediated migration: once the neural proliferation is underway and the walls of the neural tube are thickening, a temporary network of glial cells (radial glial cells) appears in the developing neural tube; most cells engaging in radial migration do so by moving along the radial glial network  Inside-out pattern: each wave of cortical cells migrates through the already formed lower layers of cortex before reaching its destination  Many cortical cells engage in long tangential migrations to reach their final destinations  The patterns of proliferation and migration are different for different areas of the cortex  Neural crest: situated dorsal to the neural tube; formed from cells that break off from the neural tube as it is being forced; neural crest cells develop into the neurons and the glial cells of the PNS o Aggregation:  Once developing neurons have migrated, they must align themselves with other developing neurons that have migrated to the same area to form the structures of the nervous system (this is called aggregation)  Migration and aggregation are thought to be mediated by cell-adhesion molecules (CAMs): located on the surfaces of neurons and other cells  CAMs recognize molecules on other cells and adhere to them  Abnormalities of CAM function result in some neurological disorders o 4 Stage – Axon growth and synapse formation:  Axon growth:  Once neurons have migrated to their positions and aggregated into neural structures, axons and dendrites begin to grow from them  Growth cones: axons grow via growth cones that follow a certain route to reach their target  Pioneer growth cones: follow a trail by interacting with guidance molecules along the route; then, subsequent growth cones following the same path go along the routes left by the pioneers  Fasciculation: the tendency of developing axons to grow along the paths established by preceding axons  Synapse Formation:  Once axons have reached their intended sites, they must establish an appropriate pattern of synapses  Synaptogenesis: the formation of new synapses  ^this depends on the presence of glial cells (particularly astrocytes) – ex. retinal ganglion cells maintained in culture formed seven times more synapses when astrocytes were present and the synapses formed in the presence of astrocytes were quickly lost when the astrocytes were removed  A lot of cholesterol is required for developing neurons during th synapse formation (extra cholesterol is supplied by astrocytes) o 5 Stage – Neuron death and synapse rearrangement:  Neuron death  Genetic programs inside neurons are triggered and cause them to actively commit suicide  Necrosis: passive cell death  Apoptosis: active cell death  Apoptosis is safer than necrosis  Necrotic cells break apart and spill their
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