chap 9- development of ns
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Neuroplasticity
Development of the Nervous System
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Between 4 weeks and 9 months the brain
undergoes rapid development
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Development of the Brain
Stages of neuroanatomical development
a. Zygote stage: Begins upon fertilization of
the ovum
i. 2 sets of 22 chromosomes, and one set of
sex chromosomes. Total of 23 pairs of
chromosomes (XX= female, XY= male)
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ii. 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 ofblastocyst
(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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From Conception to 8 days
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b. Embryonic stage (14 days 8 weeks):Begins at the full formation of embryonic disc
i. Mesoderm: contains chemical signals forvarious areas of nervous system; directsformation
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
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Simplified View of Neural Plate Formation
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Formation of the Neural Plate with the
thickening of Ectoderm
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Formation of the Brain, Spinal Cord and PNS
(Neuroectoderm)
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iv. Neuroectoderm forms when foundation for
the three main structures has been developed:
1. 3 swellings
a. Hindbrain
b. Midbrain
c. Forebrain2. Closed Neural Tube becomes spinal cord
central canal and ventricles of the brain
v. Spina bifida Neural Tube Defect (NTD) -
spinal cord doesnt close, often linked to
mental retardation
**NTDs can be discovered in utero **
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vi. Amount of mesoderm in relation to ectodermdetermines the nervous system region
1. Amount of signal/tissue coming frommesoderm determines spinal cord/brain
Determination: A process that ensures that a
population of cells will give rise to specificsystems in developing organism
Regional specificity: Once signal is in placewe lose the plasticity of the blastocysts.Irreversible signal set by a genetic code afterformation of neuroectoderm.
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Ectoderm to Neuroectoderm: 20 days to 30 days
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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
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Neuronal Proliferation(embryonic stage through fetal stage)
Midbrain Ventricular Intermediate Marginal
Hindbrain Zone Zone Zone
Forebrain Ventricul. SubV Intermed. Cortical Marginal
Zone Zone Zone Plate Zone
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Neuronal Proliferation
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Six Stages of Nervous System Development
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1.Mitosis (cell division) orNeurogenesis in the
ventricular zone, One cell division can lead to a daughtercell, will divide again forming an immature neuron
2.Migration: cells move from VZ to their destination;
this migration is aided by glial cells, abnormal migrationfound in a number of disorders. Filopodia assist in findinglocation after leaving radial glial cells.
Abnormalities in migration are presentin people with learning disabilities,
schizophrenia and autism (more on this
in a little while)
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Cell Migration During
Brain Development: Hindbrain & Midbrain
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Forebrain Development
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Growth Cones &F
ilopodia
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Filopodia
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3. Differentiation: The Process which gives
rise to specific 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 NTNeurotrophic factors stimulate cell growth,i.e. nerve growth, factor helps neuron tomature.
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5. Normal cell death (more on this later)
- Apotosis active cell death duringdevelopment
- Necrosis passive cell death due to injury
6. Synaptic rearrangement: dependent on
apotosis and experience!!!!
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Theories as to why/how this happens
1. Chemoaffinity hypothesis Postsynaptic cell is releasing a chemical
2. Blueprint hypothesis Cell adhesionmolecule present, guides neuron to
destination3. Topographic gradient hypothesis
Axons are growing based on position ofcell body, spatial growth
**All three appear correct, happensdifferently in different areas**
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Postnatal Development of the
Central Nervous System
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Post-natal brain development
1. At birth the brain weighs 25% of the fulladult brain
2. By the age of 6 it increases to 95%
i. Increase is due to myelination
a. At birth the brain is myelinatedthrough the thalamus
b. Myelination is in part based on
experience (the premature baby willhave substantially more myelin than thatof the full term baby)
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ii. Proliferation of glial cells
iii. Last wave of neurogenesis
iv. Maturation of neurons
v. Increase in synaptic connectivity
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Cellular Development of the
Postnatal Brain
Cells of the Cerebral Cortex
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Cell of the
Cerebellum
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Brain Development Occurs in
Waves through Age 21 Temporal
Parietal
Limbic
Frontal
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Brain Increases Connections
Between Birth and 21 years old.
Age
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Changes from Birth 21:
Temporal Lobes or LanguageAreas
Age
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Changes from Birth 21:
Parietal-Temporal Areas for HigherCognitive Functioning
Age
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Changes from Birth 21:
Limbic System for
Emotions/Attachment
Age
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Changes from Birth 21:
Frontal Lobes forBehavioral
Control
Age
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VII.H
ow experience affects development1. Neural activity regulates gene expression that
directs synthesis of cell adhesion molecules
2. Neuronal activity regulates the release of
neurotrophins (NGF) that are released from thedendrites; after synaptic connectivity
3. Stimulates foundation NT and this promotes
subsequent development
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Experience can Modify Brain
Cell Connections
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Active Cell Death 40% occurs during thefirst 2 years of life, and it occurs on adifferent scale in adolescence (hormonerelated, final sculpting
1. Essential because many cells are
unconnected and useless2. Dysfunction in apoptosis is seen in post-
mortem brains of children with autism(particularly problems in cerebellum,
midbrain, & hippocampus); insufficienthooking up of neurons
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Removal ofBrain Cells during
Early Childhood
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Brain Cells can Continue to
Develop in Abnormal Ways
Early
Childhood
Adulthood
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Example of
normal and
abnormal cellconnections and
arrangements
that can occur
in differentbrain systems.
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