OPTION A NEUROBIOLOGY
A1 NEURAL DEVELOPMENT
• Essential idea: Modification of neurons starts in the earliest stages of
embryogenesis and continues to the final years of life
• Nature of science:
• Use models as representations of the real world—developmental
neuroscience uses a variety of animal models. (1.10)
• A small number of animal species is used for most of the research and
these species are known as animal models.
• Eg flatworm, zebrafish, house mouse, African clawed frog
Development of neural tube
• The neural tube of embryonic chordates is formed by infolding of
ectoderm followed by elongation of the tube.
Neurulation
• The process of development of dorsal nerve cord is neurulation.
• In human this occurs in first month of life.
• In gastrula an area of ectodermal cells develop into neural plate.
• Cells in neural plate change shape and this causes plate to fold
inwards forming a groove along the back of the embryo and then
separating from the rest of the ectoderm when the edges of the
groove fuse.
• This forms neural tube that develops into nerve cord.
Development of CNS
PNS
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Nervous system
CNS
Both spinal cord and brain develop from neural tube.
As neural tube grows embryo elongates.
Anterior part of neural tube develops into brain and rest thickens to form
the spinal cord.
• Channel in centre of neural tube persists as neural canal in spinal cord.
• For more neurons, cell proliferation continues in developing spinal cord
and brain.
• Although this ceases before birth in most parts of NS, there are parts of
brain where extra neurons are produced during adulthood.
No need to learn these terms
Development of neurons
• During embryonic development part of the ectoderm develops into
neuro ectodermal cells in neural plate.
• Nervous system develops from these cells.
• These neuro ectodermal cells continuously divide to form neuroblasts
which are immature nerve cells.
• Neurogenesis is the process of differentiation from neuroblasts to
neurons.
Migration of neurons
• Both brain and spinal cord develop from the neural tube. How?
• Some immature neurons migrate from where they are produced in
the neural tube to final location.
• As soon as neural tube begins to form specific brain parts, cells
differentiate to form two types of cells – neurons and glial cells
• 90% brain is glial cells. They give physical and nutritional support to
neurons.
• Along the scaffolding of glial cells, immature nerve cells migrate to
final location and mature sending out axons.
• Cytoplasm and organelles of immature neurons are moved from the
trailing end of the neuron to leading end by contractile actin
filaments.
Development of Axons
• Immature neuron has cell body, cytoplasm and nucleus
• Chemical stimuli determine when axon grows from cell body and
direction of growth. (after reaching the target organ)
• The target cells give this chemical signals.
• Eg. CAM to the neuron
• This signal molecule (chemical signal) from target cell can
a. Be secreted into extracellular environment, or
b. Carried on target cell surface
Chemical messages on target cells
• Axons grow at the edges.
• Certain molecules from the target cells can act as signals to the
growth cone. How?
• CAM (cell adhesion molecule) is a signal molecule. It is located on the
surface of cells present in the growth environment of the axon.
• Growth cone of axon has a CAM specific receptor, so that when CAM
and its receptors recognize each other. Chemical messaging takes
place within the neuron.
• This results in activation of enzymes within the neuron that
contribute to the elongation of the axon.
Chemicals from target cells diffusing into
extracellular environment
• These are called chemotrophic factors. They are of two types:
a. Chemoattractive factors – attract the axons to grow towards it
b. Chemorepellent factors – repel axon so that it grow in the opposite
directions.
Growth of axons
• Some axons are short making connections between other neurons in
CNS
• Some axons extend beyond neural tube to reach other parts of
embryo forming connections with either muscle or gland cell. These
neuron develop into a sensory or motor neuron.
• An axon can grow about 1mm per day.
What should a muscle or gland produce for this?
Formation of synapses.
• When neurons have reached their final location synaptic connections
must be made with the target cells.
• Target cells produce chemical messages. This signal molecule from target cell
can
a,. Be secreted into extracellular environment, or
b. Carried on target cell surface
• Neuron responds to these chemical messages by forming synapses with target
cells.
Multiple synapses
• A single nerve cell has many points of branching that forms many
connections (synapses) with neighbouring nerve cells or other cells.
• Only those synapses that have a function will survive and others will
disappear.
• These rapid connections is controlled by a by a type of neural adhesion
molecule called immunoglobulin CAM.
• This forms a physical but glue like bond between the tentative projection of
one cell’s axon and receiving structure of neighbouring axon.
• Eventually many connections are lost because they are not the right
partners.
Elimination of synapses
• When transmission occurs at a synapse, chemical markers are left
that cause the synapse to be strengthened .
• Inactive synapses do not have these markers , become weaker and
are eventually eliminated.
• “use it or lose it”
• Purpose is to remove simple connections and replace them with more
complex wiring made in adults. Improves brain efficiency.
• Cells called microglia can prune unused synapses.
Neural pruning
• Involves the loss of unused neurons – a part or the whole cell
Evidence – more neurons in babies than adults
Neurons not used destroy themselves by apoptosis.
Plasticity of nervous system
• This allows nervous system to change with experience.
• This ability of nervous system to rewire its connections is called
plasticity.
• It continues throughout life but higher degree of plasticity is upto the
age of six rather than later.
• Stimulus for change in connections comes from – a person’s
experience and how their nervous system is used.
• Plasticity is the basis for forming new memories and also for certain
forms of reasoning.
• Also important in repairing damage to brain and spinal cord.
Stroke
• Disruption of blood supply to a part of the brain.
• Causes:
Blood clot blocking one of the small blood vessels.
Bleeding from blood vessel.
• Such strokes promote reorganization of nervous system (linked to
plasticity)- functional
SPINA BIFIDA
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Each vertebra has a strong centrum and a
thinner vertebral arch, which encloses
the spinal cord.
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Centrum is in the ventral side of neural
tube at early stage of development.
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Tissues migrate from both sides of
centrum around the neural tube and
meet up to form the vertebral arch.
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In some cases the two parts of the arch
never fuse together properly leaving a
gap leading to spina bifida.
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Common in lower back.
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Varies from mild to severe to debilitating.
Spina bifida
Structural plasticity