DEVELOPMENT OF CNS

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DEVELOPMENT OF CNS

• Lecture 1

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Formation of neural tube

• At the beginning of the third week of
  development, the ectodermal germ layer has the
  shape of a disc that is broader in the cephalic
  than the caudal region .

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• Appearance of the notochord and prechordal
  mesoderm induces the overlying ectoderm to
  thicken and form the neural plate .

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• Appearance of the notochord and prechordal
  mesoderm induces the overlying ectoderm to
  thicken and form the neural plate .

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By the end of the third week, the lateral edges
of the neural plate become more elevated to form
neural folds, and the depressed mid region forms
the neural groove
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• Gradually, the neural folds approach each other
  in the midline, where they fuse .

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• cells along the margin of the neural groove is
  called the neural crest
• develop into sensory and sympathetic neurons and
  schwann cells

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• Fusion begins in the cervical region and proceeds
  cranially and caudally . As a result, the neural
  tube is formed. Until fusion is complete, the
  cephalic and caudal ends of the neural tube
  communicate with the amniotic cavity by way of
  the cranial and caudal neuropores, respectively

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• Neurulation is then complete, and the central
  nervous system is represented by a closed tubular
  structure with a narrow caudal portion, the
  spinal cord, and a much broader cephalic portion
  characterized by a number of dilations,
• the brain vesicles

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• The wall of a recently closed neural tube
  consists of neuroepithelial cells.
• These cells extend over the entire thickness of
  the wall and form a thick pseudostratified
  epithelium . Junctional complexes at the lumen
  connect them. During the neural groove stage and
  immediately after closure of the tube,they divide
  rapidly, producing more and more neuroepithelial
  cells. Collectively they constitute the
  neuroepithelial layer or neuroepithelium.

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• Once the neural tube closes, neuroepithelial
  cells begin to give rise to another cell type
  characterized by a large round nucleus with pale
  nucleoplasm and a dark-staining nucleolus. These
  are the primitive nerve cells, or neuroblasts.
  They form the mantle layer, a zone around the
  neuroepithelial layer . The mantle layer later
  forms the gray matter of the spinal
• cord.

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• The outermost layer of the spinal cord, the
  marginal layer, contains nerve fibers emerging
  from neuroblasts in the mantle layer. As a result
  of myelination of nerve fibers, this layer takes
  on a white appearance and therefore is called the
  white matter of the spinal cord

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BASAL, ALAR, ROOF, AND FLOOR PLATES

• As a result of continuous addition of neuroblasts
  to the mantle layer, each side of the neural tube
  shows a ventral and a dorsal thickening.

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• The ventral thickenings, the basal plates, which
  contain ventral motor horn cells, form the motor
  areas of the spinal cord the dorsal thickenings,
  the alar plates, form the sensory areas .

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• A longitudinal groove, the sulcus limitans, marks
  the boundary between the two.

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• The dorsal and ventral midline portions of the
  neural tube, known as the roof and .floor plates,
  respectively, do not contain neuroblasts they
  serve primarily as pathways for nerve Fibers
  crossing from
• one side to the other.

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• In addition to the ventral motor horn and the
  dorsal sensory horn, a group of neurons
  accumulates between the two areas and forms a
  small intermediate horn . This horn, containing
  neurons of the sympathetic portion of the
  autonomic nervous system, is present only at
  thoracic (T1T12) and upper
• lumbar levels (L2 or L3) of the spinal cord.

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HISTOLOGICAL DIFFERENTIATION

• Nerve Cells
• Neuroblasts, or primitive nerve cells, arise
  exclusively by division of the neuroepithelial
  cells. further development becomes the adult
  nerve cell or neuron.

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• Axons of neurons in the basal plate break through
  the marginal zone and become visible on the
  ventral aspect of the cord. Known collectively as
  the ventral motor root of the spinal nerve, they
  conduct motor impulses from the spinal cord to
  the muscles .

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• Axons of neurons in the dorsal sensory horn (alar
  plate) behave differently from those in the
  ventral horn. They penetrate into the marginal
  layer of the cord, where they ascend to either
  higher or lower levels to form association
  neurons.

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• Glial Cells
• The majority of primitive supporting cells,
  the gliablasts, are formed by neuroepithelial
  cells and migrate from the neuroepithelial layer
  to the mantle and marginal layers. In the mantle
  layer, they differentiate into protoplasmic
  astrocytes and .fibrillar astrocytes
• Another type of supporting cell possibly derived
  from gliablasts is the oligodendroglial cell.
  This cell, which is found primarily in the
  marginal layer, forms myelin sheaths around the
  ascending and descending axons in the marginal
  layer.

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• In the second half of development, a third type
  of supporting cell, the microglial cell, appears
  in the CNS.
• Neural Crest Cells
• During elevation of the neural plate, a group
  of cells appears along each edge (the crest) of
  the neural folds Crest cells migrate laterally
  and give rise to sensory ganglia (dorsal root
  ganglia) of the spinal nerves and other cell
  types

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Spinal Nerves

• Motor nerve fibers begin to appear in the fourth
  week, arising from nerve cells in the basal
  plates (ventral horns) of the spinal cord. These
  fibers collect into bundles known as ventral
  nerve roots and Dorsal nerve roots form as
  collections of fibers originating from cells in
  dorsal root ganglia (spinal ganglia).

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Nerve cells in anterior gray column

• nerve cells are large multipolar known as alpha
  which innervate skeletal muscle and gamma
  efferents which innervate intrafusal fibers of
  neuromucula spindles.

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Nerve cells in posterior gray column

• At apex-substantia gelatinosa
• Nucleus prorius
• Nucleus dorsalis-c8-L4

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• The dorsal region of the gray matter, called the
  dorsal or posterior horn, is associated with the
  incoming (afferent) dorsal root, and is thus
  related to sensory functions.
• The cell body of these sensory fibers is
  located in the
• dorsal root ganglion .
• The dorsal horn is quite prominent in this
  region because of the very large sensory input
  to this segment of the cord from the upper limb,
  particularly from the hand.

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• The ventral gray matter, called the ventral or
  anterior horn, is the motor portion of the gray
  matter. The ventral horn has the large motor
  neurons, the anterior horn cells,which are
  efferent to the muscles . These neurons, because
  of their location in the spinal cord, which is
  below the brain, are also known as lower motor
  neurons.