Chap 9- Development of NS

8/6/2019 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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Parietal-Temporal Areas for HigherCognitive Functioning

Age


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Limbic System for

Emotions/Attachment

Age


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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.

Documents

Chap 9- Development of NS