Download Neurons are initially produced by differentiation in the neural tube.

Option A – Neurobiology and behavior
Essential idea: Modification of neurons
starts in the earliest stages of
embryogenesis and continues to the final
years of life.
Embryology – The study of development of organisms from a single, fertilized cell.
The study of embryology
encompasses the
development of gametes,
fertilization and the
development of embryos
and fetuses.
This process aims to
understand development in
humans, and as such uses
the most close
approximations for human
subjects in experimentation
– organisms from the
phylum Chordata.
?
1800
Time
1900
Scientists began using frog
embryos as a model
organism, but around 1900,
began to use chick embryos
instead, due to their closer
evolutionary relationship
with humans.
One of the benefits of these studies is that we now have a much
stronger understanding of the neural development of embryos in
general.
Early embryonic development is a result of many mitotic divisions and cell specialization.
Annotation of a diagram of embryonic tissues in Xenopus, used as an animal model, during
neurulation.
First thing about development – All structures
appear in a specific order, and receive signals
from previously established tissues.
After fertilization of the egg in a frog embryo,
three tissue layers develop. These go on to
form different tissues in the adult frog –
Ectoderm
Brain
Nervous
Endoderm
Lining of gut and other organs
Mesoderm
Skeletal
Reproductive
Circulatory
Excretory
Muscular
Embryonic tissue of Xenopus
Identify these tissues on the diagram
above. Note the placement of each with
respect to the whole embryo.
Try and draw this tissue without using
your notes!
The neural tube of embryonic chordates is formed by infolding of ectoderm followed by
elongation of the tube.
At approximately 3 weeks into
development, the growing embryo
begins to develop the nervous
system from the ectoderm.
3 Weeks
Time
This infolding of the
ectoderm will eventually
lead to the formation of
the different portions of
the nervous system.
8 Weeks
The notochord will eventually disintegrate and
only compose our intervertebral discs.
The neural crest will eventually become portions
of our peripheral nervous system (PNS).
Neural canal becomes spinal cord.
Neurons are initially
produced by
differentiation in the
neural tube.
Incomplete closure of the embryonic neural tube can cause spina bifida.
Spina bifida is a disorder in which the
neural tube forms incompletely, most
often in the lumbar region of the spine.
X
Great series on Spina bifida and its implications for parents.
This leads to a small protrusion on
the lower back of the baby.
The potential ramifications are:
Fluid in the brain
Mental Retardation
Growth abnormalities
Loss of control of autonomic
functions
Fortunately, the symptoms can now
be treated often with surgery after
birth. (Or even before in some cases)
https://www.youtube.com/watch?v=6Ii_v3t9hpU
Immature neurons migrate to a final location.
The neurons in the developing embryo move to
where they will be used through a process
known as neuronal migration.
Cells formed earlier in neuronal
development migrate small distances
using glial fibers as a scaffold on which
to climb.
Neurons created later in the process
will move much more distance, which
Brains at different stages of development - http://ventricular.org/
causes the brain to add on to itself in
layers as it develops.
Animation
Live Feed
Check out the clips to the left
to see just how hard these
neurons work to get where
they are needed in the brain!
An axon grows from each immature neuron in response to chemical stimuli.
These axons grow to form
the different regions of the
brain, and form synapses
with one another.
Many times the neurons
form multiple synapses as
they grow.
Some axons extend beyond the neural tube to reach other parts of the body.
These axons go on to form portions of the
peripheral nervous system, and are
recruited by signals.
To the right, see how cell-adhesion
molecules (CAMs) induce the formation of
a synapse with tissue away from the
nervous system.
How is this an
application of
chemotaxis?
Synapses that are not used do not persist.
Much in the same way that you
try all of your possible passwords
when you forget which one you
used, the developing neurons try
all available synapse formations
with those neurons around them.
These connections are moderated by CAMs, and are
held in place once determined favorable.
X
Neural pruning involves the loss of unused neurons.
However, these synapses do not stick around!
Synapses that are not used during this critical
period are degenerated during neural pruning.
At age 2-3, you have twice the amount you
have in adolescence and adulthood!
This partly has to do with how adult synapse
formation is a much more complex.
The plasticity of the nervous system allows it to change with experience.
Sweet TED Talk on Brain Scans
Modern research has proven that experience
changes the brain, and has shown that the
brain exhibits both functional and structural
plasticity.
Structural Plasticity
Remodeling of synaptic elements e.g. spine
strengthening/weakening, taxi drivers have enlarged
hippocampus
Functional Plasticity
Recovery of function, due to existing neurons moving to fill
in the gaps left behind, e.g. learning to use arm after
stroke
Events such as strokes may promote
reorganization of brain function.
For a long time, we treated people
with brain injuries as non-functioning
people, and there is currently a
paradigm shift to now refer to these
people as being in a dormant state,
implying that they can be awoken,
given the proper stimulus.
http://www.frontiersin.org/files/Articles/135655/fnhum-09-00394HTML/image_m/fnhum-09-00394-g004.jpg
Essential Idea: Modification of neurons starts in
the earliest stages of embryogenesis, and
continues to the final years of life.
Challenge Questions
After this
subunit, can
you do each
of these?
Redraw the picture of Xenopus
from memory to include each of
the tissue types.
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Describe spina
bifida.
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Annotate the
table of tissues in
Xenopus with the
eventual fate in
the adult frog.
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Outline the differentiation and
migration of immature neurons.
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Compare and contrast
functional and structural
plasticity of the brain.
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Explain neural
pruning.
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