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The Nervous System
SMS 1084
Dr. Mohanad R. Alwan
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The Nervous System
SMS 1084
Dr. Mohanad R. Alwan
12 June 2007 CNS1.ppt 2
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Stages and Time Frames
Ovum
Zygote
Morula
Blastocyst Embryo
Fetus
(Oh, Zee my baby eats fast)
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Stages of fertilization
(1) 12-30 hours first cell division
(2) 3 days division continues until a solid ball is formed
(3) 5 days continue to divide, but cells move toward
outer edge of blastocyst(4) 6 days cells begin to move inward
(5) 8 days beginning of embryonic disc & zygote
becomes attached to the uterus
(6) 14 days zygote is completely attached to the uterus
and embryonic disc is fully formed
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Begins at the full formation of embryonic disc:
i. Mesoderm: contains chemical signals for various areasof nervous system; directs formation
ii. Endoderm: forms everything else
iii. Ectoderm: forms nervous system (brain), fingernails,hair, and skin. Changes:
1. Thickens in the middle (neural plate)
2. Groove starts to form (neural groove)
3. Groove closes to form tube (neural tube)a. At the top of canal is neural crest, forms PNS
Embryonic stage (14 days 8 weeks):
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Similar in all "Amniotes"
Blastoderm over yolk sac
three "germ layers"
Ectoderm ("outside skin")
Mesoderm ("middle skin")
Endoderm ("inner skin")
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Development of the CNS
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Nervous System Development
1) At 18 days after conception the embryo begins to
implant in the uterine wall. It consists of 3 layers of
cells: endoderm, mesoderm, and ectoderm.
Thickening of the ectoderm leads to thedevelopment of the neural plate (inserts).
2) The neural groove begins to develop at 20 days.
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Nervous System Development
Develops from Ectoderm
Rising of neural folds
Sinking of neural plate
forms neural groove Folds fuse to complete
neural tube
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Nervous System Development
Again
Neural tube forms CNS
(esp. spinal cord)
Expands anteriorly to formbrain
Peripheral nerves develop
from lateral "neural crest"
cells
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Nervous System Development
3. At 22 days the neural groove closes along the
length of the embryo making a tube.
4.A few days later 4 major divisions of the brain are
observable the: Telencephalon,
Diencephalon,
Mesencephalon, and
Rhombencephalon.
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Nervous System Development in the Human
Embryo
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Expansion of Brain
Primary
Forebrain
Midbrain
Hindbrain
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Expansion of Brain
Secondary
Forebrain
Telencephalon
Diencephalon
Midbrain
remains just midbrain
Hindbrain Cerebellum
Brain stem
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Then further differentiation into 5 secondary vesicles:
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Expansion of Brain
Final development
Telencephalon = cerebrum
Diencephalon = thalamus + hypothalamus
Cerebellum
Brain stem = Midbrain + pons + medulla
oblongata
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Expansion of Brain
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Expansion of Brain
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24-hr Chick22-23 days pf Human
Expansion of Brain
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23-26 days of Human
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Differentiation of the Neural Tube
Histological changes
1. Neural tube initially a single layer of cells: germinal
epithelium
2. Cells are called neural stem cells Neurons
Glial Cells: Myelin sheath
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Origin of PNS Cells
From neural tube: All motor neurons of somatic nervous system
Preganglionic neurons of autonomic system
From neural crest:
Sensory nerves and associated ganglia
Postganglionic neurons of autonomic system
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Neural Crest Cells
Induced by organizing cells of notochord Main functional groups:
Cranial neural crest:
Bones and connective tissue of face
Tooth primordia
Thymus, parathyroid, thyroid glands
Sensory cranial neurons
Parasympathetic ganglia and nerves Parts of the heart (cardiac neural crest)
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Neural Crest Cells
Main functional groups: Trunk neural crest:
Melanocytes
Sensory neurons
Sympathetic ganglia and nerves
Medulla of adrenal glands
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Differentiation of Neurons
Within nerve tube: Dorsal Interneurons
VentralMotor neurons
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Differentiation of Neurons
Motor neurons:
Tissues they innervate depends on:
Anterior-posterior location along the nerve tube
When the cells were born
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Fetal Period: Two Months to Birth (38 weeks)
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Fetal Period
Foundation for the entire CNS is set
6 stages of CNS development complete the
prenatal process
Si S f S l
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Six Stages of Nervous System Development
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1. Mitosis (cell division) or Neurogenesis in theventricular zone, One cell division can lead to a daughter
cell, will divide again forming an immature neuron
2. Migration: cells move from VZ to their destination; thismigration is aided by glial cells, abnormal migration found
in a number of disorders
Abnormalities in migration are presentin people withlearning disabilities, schizophrenia and autism (more onthis in a little while)
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3. Differentiation: The Process which gives rise tospecific neurons and glial cells
4. Synaptogenesis: Neuronal maturation
1. Elongation of axons (w/growth cones)
2. Establish terminals
3. Elongation of dendrites
4. Expression of NT
Neurotrophic factors
stimulate cell growth, i.e.nerve growth, factor helps neuron to mature.
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5. Normal cell death (more on this later)
- Apotosisactive cell death duringdevelopment
- Necrosis passive cell death due to injury
6. Synaptic rearrangement: dependent onapotosis and experience!!!!
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Cellular Development of the Postnatal Brain
Cells of the Cerebral Cortex
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Brain Development Occurs in Waves through
Age 21
Temporal
Parietal
Limbic Frontal
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