Transcript Document

Neural Development
• Between 10 and 20 billion neurons in the
adult cortex
• Between 50 and 200 billion glial (support)
cells
• There is great diversity in the shape,
form and interconnections of these
neurons
• Responsible for the ability of the CNS to
process information
Neural Development
• Goal of studying development
• How the diversity is achieved
• How each portion and structure of CNS is
integrated into a whole to allow coherent
functioning
• Each division and subdivision of brain has
its own structure and function
Brain Areas
• Brain Stem:
• Role in basic
attention, arousal,
and consciousness.
All information to
and from our body
passes through the
brain stem on the
way to or from the
brain.
Brain Areas
• Cerebellum:
•
•
Involved in the
coordination of
voluntary motor
movement, balance
and equilibrium and
muscle tone.
Possibly involved in
working memory.
Brain Areas
• Occipital Lobe:
• The center of our
visual perception
Brain Areas
• Temporal Lobe:
•
•
•
Involved in the primary
organization of sensory
input.
Language is also a
function, especially in
terms of verbal labels for
sensory information.
The temporal lobes are
highly associated with
memory skills.
Brain Areas
• Parietal Lobe:
• Can be divided into two
functional regions.
• The first function
integrates sensory
information to form a
single perception
(cognition).
• The second function
constructs a spatial
coordinate system to
represent the world
around us.
Brain Areas
• Frontal Lobe:
•
Involved in higherorder cognitive
abilities
•
Reasoning and
decision making
•
Also responsible for
planning
•
Pre-frontal area
involved in working
memory and decision
making
Assembly of Brain
• Genome is the blueprint for the brain
• Neurons and glial cells are the bricks and
mortar, etc.
• Axons, dendrites and synaptic
connections are the wiring for electricity
and telephone, etc.
• In terms of development, need to know
when, where, and what develops.
When
• Most postnatal brain growth occurs
within 3 to 4 years
• But changes in myelination occurs well
into aging, as late as 70-80 years
• Do things occur sequentially or
simultaneously?
When
• In terms of differentiation of the
different areas of the brain, this occurs
in the fetus.
• Also, early in the fetus, the brain is fairly
smooth, but by the time the infant is
born much of the convolutions and
invaginations have occurred
When
• After birth, myelination begins and
continues for many years
• Myelination allows speedy transmission of
signals across neurons and between neurons
• Further development of the different
brain areas continue in an inside-out
fashion (subcortical --> cortical)
Where
• Developmental subdivisions precede functional
subdivisions
• Easier to understand adult organization once the
simple developing system is understood
• The various dimensions and divisions of the CNS are
defined in the neural tube
• Development of the neural tube cavity becomes
the ventricles of the brain and canal of the cord
• Development of the neural tube wall provides an
early organization of the CNS
Neurulation (week 3-4)
Neural
Groove
Neural
Plate
Ectoderm
Surface
Ectoderm
Notocho rd
Neural
Groove
Ectoderm
Neural
Plate
Notocho rd
Neural
Tube
Neural
Crest
Notocho rd
Differentiation of Neural
Tube
• Bottom 50% becomes spinal cord
• Intermediate part becomes the brain
stem
• Top part becomes the brain
• The disproportionate growth of the top part
of the tube is called encephalization
• Differentiates into 3 vesicles
Three Vesicles
• Prosencephalon (forebrain)
• Divides into the telencephalon (cortex) and
diencephalon (thalamus and hypothalamus)
• Mesencephalon (midbrain)
• Rhombencephalon (hindbrain)
• Divides into metencephalon (pons and
cerebellum) and myelencephalon (medulla)
What
• Cell
• Cell
• Cell
• Cell
Proliferation
Migration
Differentiation
Death
Cell Proliferation
• By the end
of infancy
the volume
of neurons
has
increased
significantly
Volume (mm3)
26 yr
13 yr
11 yr
5 yr
3.75 yr
19 mo
4 mo
2 mo
2 wk
6 days
Newborn
28 wk GA
0
1500
3000
4500
Cell Proliferation
• The
density,
however,
has
decreased
Neurons/mm3(x104)
26 yr
13 yr
11 yr
5 yr
3.75 yr
19 mo
4 mo
2 mo
2 wk
6 days
Newborn
28 wk GA
0 10 20 30 40 50 60 70
Cell Migration
• Passive Cell Displacement
• New cells push old cells outward
• Short distances
• Outside-inside spatiotemporal
organization
• Active Neuronal Migration
• Long distances
• Inside-outside spatiotemporal
organization
Active Neuronal Migration
• How to neurons know how to get there
• Via Glial cells
• Neuron starts its migration
• Neuron propels itself along surface of glial
cell
• Neuron “recognizes”, by way of chemical
signals, that it has reached its final
destination and stops its migration
Cell Differentiation
• Axons and dendrites
form
sy napse/mm3
total syna pse
7
• Allows for formation of
6
synapses
5
• The number of
synapses, the synaptic 4
density, and the number3
of synapse per neuron 2
continue to increase 1
during the first year and0
28 wk
then steadily decline
GA
2 mo 8 mo
2 yr
10 y r 70 y r
Neural Development
• The number of
synapses, the
18000
synaptic density, 16000
and the number of14000
12000
synapse per
10000
neuron continue 8000
to increase during 6000
4000
the first year and 2000
0
then steadily
decline
sy napse/neuron
28 2 mo 8 mo 2 yr 10 y r 70 y r
wk
GA
Neural Development
• Stages:
• Cell Proliferation Fetus
• Cell Migration - 7
mos.
• Cell Differentiation
• Culling
• Myelination - 4 yrs.
Quic kTime™ and a
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Brain Development
• The subcortical to cortical development
of the control of behavior has been best
demonstrated via visual behavior
• Johnson (1990) suggest that newborns
visual behavior, particularly their eye
movements, are controlled by
subcortical pathways
• During the first 6 months, the cortical
pathways functionally develop so that they
can influence eye movements
Johnson’s Brain
Development
Johnson (1990)
• One particular hypothesis concerned anticipatory
eye movements
• Required the functioning of mechanisms within the frontal
cortex
• Therefore, should not see anticipatory eye movements
before approximately 20 weeks of age
• Recent results by Haith, Hazan & Goodman (1988),
Canfield & Smith (1996), and Adler & Haith (2003)
indicate that infants as young as 12 weeks exhibit
anticipatory eye movements
• Indicates that frontal cortex is functional earlier than
believed
Visual Expectation Paradigm
Eye Movement Classification
-1000
msec
QuickTime™ and a Video decompressor are needed to see this picture.
+167
msec
Anticipation
Window
+867
msec
Reactive
Eye
Movement
Window
Time
0 msec
Stimulus
Onset
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700 msec
Stimulus
Offset
VExP Design and Findings
Percent Anticipations
25
20
15
10
5
0
Median Reaction Times (milliseconds)
Haith, Hazan, & Goodman (1988)
Design
700
650
600
550
500
450
400
350
300
250
200
Alternating
Irregular
Baseline
Post-Baseline
Alternating
Irregular
Spatial Condition