Download Neural Development

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Neuroscience is all about understanding how
the nervous system works and is formed from
embryo to adults.
Understanding can lead to the treatment of
various diseases of the nervous system.
However, because our nervous systems are
quite complex and because of ethical
reasons, neuroscientists often study the
nervous systems of animals
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Flatworm – they mature quickly and have a
low, fixed number of cells as an adult
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Fruit Fly – breed easily, mature quickly, and
only 4 chromosomes
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Zebrafish – tissues practically transparent
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American clawed frog – large eggs, easily
manipulated
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House mouse – shares many human diseases
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Neurulation: the process in which
chordates* develop a dorsal nerve cord in the
early stages of development.
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For humans, it occurs during the 1st month of
pregnancy.
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*Chordates: a phylum that humans belong to
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An area of the ectoderm develops
into the neural plate
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The neural plate folds inwards
forming a groove along the back
of the embryo and separates from
the rest of the ectoderm
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This is the neural tube that
elongates as the embryo grows
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The space inside will be the centre
of the spinal cord
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Annotate a diagram of embryonic tissues in
Xenopus laevis (African clawed frog)
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See page 515
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A condition caused by the incomplete closure
of the embryonic neural tube.
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It usually occurs in the lower back.
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Can be mild (with no apparent symptoms –
severe (with debilitating symptoms that can
lower ability to walk, result in problems with
bladder and bowel control)
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There are billions of neurons in your body.
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As the neural tube is produced, cells undergo
mitosis and differentiation occurs leading to
cells turning into neurons.
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Proliferation continues as the spinal cord and
brain develop.
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MYTH: You are born with all the neurons you
will ever have. You do not produce more
neurons after birth
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Cell division stops in most parts of the
nervous system before birth, however, there
are may parts of the brain where new
neurons are produced during adulthood!
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Migration of neurons in important in brain
development.
Some neurons are produced in one parts of the
developing brain need to migrate to another
part.
The cytoplasm and organelles in it are moved
from one end of the neuron to anther (similar to
how an Amoeba moves with contractile actin
filaments)
https://www.youtube.com/watch?v=ZRFgKZHINk
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Mature, functional neurons do not normally
move, though their axons and dendrites may
regrow if damaged
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An immature neurons contains a cell body
with a cytoplasm and nucleus.
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An axon is a long extension of the cytoplasm
from the cell body which carries nerve
impulses to another neuron.
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Only one axon develops on each neuron
(though it can be highly branched at the end
plate)
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Chemical stimuli determine neuron
differentiation.
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(Dendrites are shorter and numerous and
bring impulses from other neurons to the cell
body)
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Axons grow at their tips
Within the CNS they are short and make
connections between other neurons in the
CNS
Some neurons within the PNS develop very
long axons (1m+ in humans) and take
impulses to other neurons or effector cells
(muscles or glands)
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Axons can me many meters long in some
animals
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As long as the cell body of the neuron
remains intact, the axon may be able to
regrow if damaged or severed outside the
CNS.
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Can be as rapid as 5 mm /day
Control of muscles and return of sensation
may occur with time after the initial injury.
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A developing neuron forms multiple
synapses.
The minimum is 2 (one to bring impulses in at
the dendrites, the other to pass them on at
the axon end plate)
Most neurons develop multiple synapses
(some in the brain develop hundreds)
allowing for complex patterns of neural
communication.
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Synapse development involves special
structures being assembled in the
membranes on either side of the synapse and
in the synaptic cleft between them
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Synapses often disappear if they are not used.
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When synaptic transmission occurs, chemical
markers are left that cause the synapse to be
strengthened. (ie: increasing the number of
receptors on post synaptic membrane)
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Inactive synapses will not have these markers
and will become weaker and are eventually
eliminated.
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Neural Pruning: the elimination of a neuron
(or part of it)
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There are more neurons in some parts of
newborn babies brains than in adults.
This indicates that some neurons are lost
during childhood.
Neurons that are not used destroy
themselves by apoptosis.
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New born babies have
~11.2 million neurons in the
thalamus
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Adult humans have ~6.43
in the thalamus
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The nervous system changes with
experiences
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Connections between neurons can be
changed by
 growth of axons and dendrites
 Formation of new synapses
 Pruning of dendrites, axons, or whole neurons
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Plasticity continues throughout life, but much
higher from birth to age 6.
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Plasticity occurs because of experiences and
the way the nervous is utilized by the
individual
It’s the basis for forming new memories and
reasoning.
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During a stroke, part of the brain is deprived
of sufficient oxygen and glucose.
Neurons may become irreparably damaged
and die.
Minor strokes may be hardly noticed.
Of major strokes, 1/3 will full recover, 1/3
survive with a disability
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Recovery from strokes will happen within the
6 months following a stroke
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It involves parts of the brain taking on new
functions to make up for the damaged areas.
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New synapses form
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In this way, a stroke patient may relearn
aspects of speech, writing, motor skills, etc.