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A.1 Neural development
Essential idea: Modification of neurons starts in the earliest
stages of embryogenesis and continues to the final years of life.
This coloured Microgrpah show the development of the neural tube. At this early
stage of development, the embryonic neural plate is still closing. Below the
neural tube is the notochord, and at each side of the neural tube the
mesoderm is organised to form the somites.
By Chris Paine
http://www.bioknowledgy.info/
http://www.ibmb.csic.es/filesusers/Fireworks%20in%20the%20NT.jpg
Understandings, Applications and Skills
Statement
A.1.U1
A.1.U2
Guidance
Terminology relating to embryonic
The neural tube of embryonic chordates is formed by
brain areas or nervous system
infolding of ectoderm followed by elongation of the tube.
divisions is not required.
Neurons are initially produced by differentiation in the
neural tube.
A.1.U3
Immature neurons migrate to a final location.
A.1.U4
An axon grows from each immature neuron in response
to chemical stimuli.
A.1.U5
Some axons extend beyond the neural tube to reach
other parts of the body.
A.1.U6
A developing neuron forms multiple synapses.
A.1.U7
Synapses that are not used do not persist.
A.1.U8
Neural pruning involves the loss of unused neurons.
A.1.U9
The plasticity of the nervous system allows it to change
with experience.
A.1.A1
Incomplete closure of the embryonic neural tube can
cause spina bifida.
A.1.A2
Events such as strokes may promote reorganization of
brain function.
A.1.S1
Annotation of a diagram of embryonic tissues in
Xenopus, used as an animal model, during neurulation.
Review: 6.5.U1 Neurons transmit electrical impulses.
Schwan
cell
A.1.U1 The neural tube of embryonic chordates is formed by infolding of ectoderm followed by elongation of the tube. AND
A.1.U2 Neurons are initially produced by differentiation in the neural tube.
Neural tube
Is a hollow structure from which the brain and spinal cord form. It runs the length
of the dorsal side of the body.
Key to neural development are the processes of cell division and differentiation, which
respectively allow the growth of the organism and an increase in complexity as relatively
unspecialised cells become increasingly specialised, e.g. ectoderm becomes the neural
tube which contains cell that eventually differentiate to become neurons, once formed the
neural tube elongates to match the growth of the embryonic organism.
http://thebrain.mcgill.ca/flash/d/d_09/d_09_cr/d_09_cr_dev/d_09_cr_dev_2a.jpg
Nature of science: Use models as representations of the real world - developmental neuroscience uses a variety of animal
models. (1.10)
Developmental neuroscience aims to discover how nervous
systems are formed during the development of adults from embryos.
Xenopus sp. (African clawed frog) because the
eggs are large and easily manipulated
Experimentation is essential and, due to the
often invasive or damaging nature of the
investigations, it is unethical to perform many
experiments in humans.
Understanding of neurological
development can improve the
understanding of diseases that
affect the nervous system and
hence bring science closer to
treatments and cures.
Animal models are useful in research
for several key reasons:
• Less ethical concerns
• Nervous systems less complex –
easier to observe
• Development is quicker
• External development of the
embryo – easier to observe
https://commons.wikimedia.org/wiki/File:Krallenfrosch_Xenopus_laevis.jpg
A.1.S1 Annotation of a diagram of embryonic tissues in Xenopus, used as an animal model, during neurulation.
Annotate the diagrams to show the process of neurulation in Xenopus
Xenopus is a genus of frogs native to sub-Saharan Africa.
Several species are commonly used as model organisms
for studies in developmental biology.
2
https://commons.wikimedia.org/wiki/File:Krallenfrosch_Xenopus_laevis.jpg
https://commons.wikimedia.org/wiki/File:2912_Neurulation-02.jpg
A.1.S1 Annotation of a diagram of embryonic tissues in Xenopus, used as an animal model, during neurulation.
Annotate the diagrams to show the process of neurulation in Xenopus
Ectoderm tissue differentiates to form
the neural plate and neural plate border.
The Notocord is dervied from the
mesoderm tissue.
Xenopus is a genus of frogs native to sub-Saharan Africa.
Several species are commonly used as model organisms
for studies in developmental biology.
2 The neural plate folds inwards and downwards. The
notocord is ‘pushed’ downwards by the folding. Eventually
the neural plate borders meet to form the neural crest.
https://commons.wikimedia.org/wiki/File:Krallenfrosch_Xenopus_laevis.jpg
https://commons.wikimedia.org/wiki/File:2912_Neurulation-02.jpg
A.1.S1 Annotation of a diagram of embryonic tissues in Xenopus, used as an animal model, during neurulation.
Annotate the diagrams to show the process of neurulation in Xenopus
https://commons.wikimedia.org/wiki/File:2912_Neurulation-02.jpg
A.1.S1 Annotation of a diagram of embryonic tissues in Xenopus, used as an animal model, during neurulation.
Annotate the diagrams to show the process of neurulation in Xenopus
The closure of the neural tube separates
the neural crest from the ectoderm. The
neural crest cells will develop
(differentiate) to form the majority of the
peripheral nervous system, e.g. the
ganglions seen in the second image.
Mesoderm differentiate to become
Somites, which will eventually give rise to
parts of the skeleton and muscle systems,
including the vertebrae.
The neural tube will eventually form the
brain and spinal cord.
The Notocord degenerates to eventually
become the intervertebral discs.
https://commons.wikimedia.org/wiki/File:2912_Neurulation-02.jpg
A.1.A1 Incomplete closure of the embryonic neural tube can cause spina bifida.
Spina Bifida – a neural tube defect
Spina bifida is caused by a gap in the
neural tube: the gap arises if
infolding of part of the neural plate,
to form the neural tube during
neurulation, is incomplete.
In vertebrates there are a
series of bones called
vertebrae which enclose
and protect the spinal
cord. The spinal cord is
formed from a region of
the embryonic neural
tube.
Spina bifida can occur at any
location along the spine and
symptoms can vary from none
to severe and debilitating.
Spina bifida can often be corrected by surgery. Most cases can be prevented if the mother
gets enough folic acid during pregnancy.
https://commons.wikimedia.org/wiki/File:Spina-bifida.jpg
A.1.U3 Immature neurons migrate to a final location.
Migration of immature neurons
This is a distinctive feature of the development
of the nervous system.
Neural migration can occur
by use of contractile actin
filaments moving the cell
and it’s organelles in a given
direction.
Migration is particularly
important in brain
development. Mature
neurons do not move, but
regrowth can occur if
dendrites or axons become
damaged.
This is a cross-sectional view of the occipital (visual) lobe of from a monkey fetus. The center
shows immature neurons migrating along glial fibers, using them as scaffolding.
http://www.brainfacts.org/~/media/Brainfacts/Article%20Multimedia/Brain%20Basics/Neuroanatomy/Neuron%20Migration.ashx
A.1.U4 An axon grows from each immature neuron in response to chemical stimuli. AND A.1.U5 Some axons
extend beyond the neural tube to reach other parts of the body.
Axon growth in immature neurons
Axons are long, narrow outgrowths
from the cell body. They carry the
impulse from neuron to neuron.
Axons grow out from each immature neuron. In some cases the axon grows out of the neural
tube to other parts of the embryo, e.g. a motor neuron needs to connect to muscle fibres. An
axon in humans maybe up to 1m long and longer in larger animals.
Chemical stimuli determine the direction and length of
the axon, i.e. differentiation neurons, during the
growth of the embryo.
Connections between neurons are highly branched:
neurons commonly possess multiple dendrites for
receiving impulses from different neurons and multiple
terminal ends for passing signals to different neurons.
http://www.cos.uni-heidelberg.de/data/m.theiss/images/GreenNeuronturned.jpg
Edited from: http://d1dvw62tmnyoft.cloudfront.net/content/joces/128/6/1241/F7.large.jpg
A.1.U6 A developing neuron forms multiple synapses.
Formation of synapses
Synapse a structure that permits a neuron to
pass an electrical or chemical signal to another
neuron or effector cell (e.g. muscle or gland).
When a growing axon reaches the cells it is intended to connect with a synapses are
developed; the synapses consists of specialised membranes from the two cells
separated by a narrow gap (cleft).
Most neurons develop multiple synapses at both the
initiating dendrites and terminal ends. This allows complex
patterns of communication.
http://understandingcontext.com/wp-content/uploads/2014/01/Synapse-Formation1.png
http://med.stanford.edu/mcp/_jcr_content/hero/hero_banner/images/imageSlide8.img.620.high.jpg
A.1.U7 Synapses that are not used do not persist. AND A.1.U8 Neural pruning involves the loss of unused neurons.
Neural pruning is the process of
synapse elimination, by retraction of the
axons from unwanted synaptic connections.
Apoptosis of neurons involves the killing
of the cell and elimination of all connections
associated with the neurons.
Many synapses are formed during fetal and early childhood development,
but new synapses can be formed at any stage of life.
"use it or lose it”: When a synapse is used it is reinforced
and unused synapses weaken. Synapses that remain unused are
eventually pruned by the retraction of the axon.
The exact role chemical stimuli in the reinforcement
and weakening of synapses is not clear.
The background image shows a tiny segment of
a human brain the lines show neurons and the
dots show synapses.
http://med.stanford.edu/mcp/_jcr_content/hero/hero_banner/images/imageSlide8.img.620.high.jpg
A.1.U9 The plasticity of the nervous system allows it to change with experience.
Neuroplasticity is a term that describes change
to the brain throughout an individual's life course.
Neuroplastic change can occur at small scales, e.g. changes to
individual neurons, large whole-brain scale, e.g. in response to injury
brain function may move to a new area of the brain.
Growth of axons and dendrites is as much a part of
neuroplasticity as pruning and apoptosis. Re-growth of
axons can be up to 5 mm per day.
Neuroplasticity is seen throughout the life of an
organism, but brain show a much higher degree
of plasticity during early childhood.
Theories explaining memory and
learning depend upon the
phenomena of neuroplasticity.
http://med.stanford.edu/mcp/_jcr_content/hero/hero_banner/images/imageSlide8.img.620.high.jpg
A.1.A2 Events such as strokes may promote reorganization of brain function.
Brain injuries and neuroplasticity
Conditions in the environment, such as social
interactions, can play a role in brain cell survival and the
formation of synapses.
In response to injury*, or disease, rearrangements in brain
function involve changes in the connection between linked
neurons.
Undamaged axons can ‘sprout’ nerve endings and connect
with damaged cells to repair damage or with undamaged
nerve cells to re-route neural pathways and brain function.
For example:
• Each brain hemisphere has it’s own tasks, if one brain
hemisphere is damaged, the other hemisphere
sometimes can take over the functions of the damaged
one (e.g. hand control).
• The brain can also respond to a lack of sensory input by
enhancing the processing of other sensory inputs, e.g.
blind individuals use areas of the visual cortex to process
hearing or touch.
*During a stroke, a blood clot or bleeding disrupts the supply of blood to a part of
the brain depriving it of oxygen and glucose. If cell respiration ceases for long
enough the neurons become damaged and die.
https://d1gqps90bl2jsp.cloudfront.net/content/brain/134/5/1264/F1.large.jpg
Bibliography / Acknowledgments
Bob Smullen