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OBJECTIVES

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  • 1) Differentiate neurogenesis and gliogenesis

  • 2) Compare and contrast neurogenesis in development and adults

  • 3) Examine mechanisms of injury to the CNS

  • 4) Differentiate the mechanisms of plasticity in the intact and injured CNS

  • 5) Differentiate adaptive and maladaptive plasticity

  • 6) Examine the role of rehabilitation in plasticity post-CNS injury

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DEVELOPMENTAL NEUROGENESIS

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As described in Chapter 18 (Neural Tube Disorders), the nervous system arises from neural crest cells within the ectodermal layer of the developing embryo. Cell proliferation transforms the neural plate into the neural tube that is initially differentiated into three vesicles from rostral to caudal: prosencephalon, mesencephalon, and rhombencephalon. Further cell proliferation results in a five-vesicle structure: telencephalon (cerebral hemispheres), diencephalon (retina, hypothalamus, thalamus, epithalamus, and subthalamus), mesencephalon (midbrain), metencephalon (pons, cerebellum), and myelencephalon (medulla), which is contiguous with the remainder of the neural tube that forms the spinal cord. The lumen of the neural tube develops into the ventricular system and central spinal canal.

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Initially, the tissue of the neural tube expands into three layers (see Figure 8-1): ependymal layer, mantel layer, and marginal layer. The ependymal layer provides the border for the developing ventricular system and is cell rich with rapidly dividing cells. The mantel layer will become the gray matter (cortex and deep nuclei), and the marginal layer will be filled with axons (white matter). Within weeks, there will be five identifiable layers: (1) ventricular zone – area of rapid cell proliferation; (2) subventricular zone – second area of cell proliferation; (3) intermediate zone – developing white matter; (4) subplate – early neurons and glial cells that facilitate connections between the cortex and thalamus;1 (5) cortical plate – developing cortex; and (6) marginal zone – cell poor with surface neurons and horizontal fibers.2

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FIGURE 8-1

Cell proliferation in the developing CNS. A. Three layered neural tube with cell proliferation in the ependymal layer and cell poor mantel and marginal layers. B. Five layers of developing brain: ventricular and subventricular zones are areas of cell proliferation filled with neural stem cells; intermediate zone will be filled with glial cells and become the white matter; the subplate will house early neurons that will connect the cortex to the thalamus; the cortical plate will become the cortex ; the marginal zone is cell poor but will contain axons and a few neurons. (Used with permission of Deborah S. Nichols Larsen, PT, PhD. The Ohio State University.)

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Cell Proliferation

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Neurogenesis refers to the development of neurons, while gliogenesis is the development of glial cells; these are interrelated processes. Neural stem cells (NSCs) are progenitor cells for both neurons and glia that can be found initially in the ventricular zone and, then, also in the subventricular zone, where they undergo rapid cell divisions. ...

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