bioknowledgy a.1 neural development
TRANSCRIPT
Essential idea: Modification of neurons starts in the earliest stages of embryogenesis and continues to the final years of life.
A.1 Neural development
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 SkillsStatement Guidance
A.1.U1 The neural tube of embryonic chordates is formed by infolding of ectoderm followed by elongation of the tube.
Terminology relating to embryonic brain areas or nervous system divisions is not required.
A.1.U2 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.
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
http://thebrain.mcgill.ca/flash/d/d_09/d_09_cr/d_09_cr_dev/d_09_cr_dev_2a.jpg
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.
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.
Understanding of neurological development can improve the understanding of diseases that affect the nervous system and hence bring science closer to treatments and cures.
Experimentation is essential and, due to the often invasive or damaging nature of the investigations, it is unethical to perform many experiments in humans.
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
Xenopus sp. (African clawed frog) because the eggs are large and easily manipulated
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.
https://commons.wikimedia.org/wiki/File:2912_Neurulation-02.jpghttps://commons.wikimedia.org/wiki/File:Krallenfrosch_Xenopus_laevis.jpg
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
Annotate the diagrams to show the process of neurulation in Xenopus
A.1.S1 Annotation of a diagram of embryonic tissues in Xenopus, used as an animal model, during neurulation.
https://commons.wikimedia.org/wiki/File:2912_Neurulation-02.jpg
Ectoderm tissue differentiates to form the neural plate and neural plate border.The Notocord is dervied from the mesoderm tissue.
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
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
Annotate the diagrams to show the process of neurulation in Xenopus
A.1.S1 Annotation of a diagram of embryonic tissues in Xenopus, used as an animal model, during neurulation.
https://commons.wikimedia.org/wiki/File:2912_Neurulation-02.jpg
Annotate the diagrams to show the process of neurulation in Xenopus
A.1.S1 Annotation of a diagram of embryonic tissues in Xenopus, used as an animal model, during neurulation.
https://commons.wikimedia.org/wiki/File:2912_Neurulation-02.jpg
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.
A.1.A1 Incomplete closure of the embryonic neural tube can cause spina bifida.
Spina Bifida – a neural tube defect
https://commons.wikimedia.org/wiki/File:Spina-bifida.jpg
Spina bifida can often be corrected by surgery. Most cases can be prevented if the mother gets enough folic acid during pregnancy.
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 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.
Spina bifida can occur at any location along the spine and symptoms can vary from none to severe and debilitating.
A.1.U3 Immature neurons migrate to a final location.
Migration of immature neurons
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
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.
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
http://www.cos.uni-heidelberg.de/data/m.theiss/images/GreenNeuronturned.jpgEdited from: http://d1dvw62tmnyoft.cloudfront.net/content/joces/128/6/1241/F7.large.jpg
Chemical stimuli determine the direction and length of the axon, i.e. differentiation neurons, during the growth of the embryo.
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.
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.
Axons are long, narrow outgrowths from the cell body. They carry the impulse from neuron to neuron.
A.1.U6 A developing neuron forms multiple synapses.
Formation of synapses
Most neurons develop multiple synapses at both the initiating dendrites and terminal ends. This allows complex patterns of communication.
http://med.stanford.edu/mcp/_jcr_content/hero/hero_banner/images/imageSlide8.img.620.high.jpg
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).
http://understandingcontext.com/wp-content/uploads/2014/01/Synapse-Formation1.png
A.1.U7 Synapses that are not used do not persist. AND A.1.U8 Neural pruning involves the loss of unused neurons.
http://med.stanford.edu/mcp/_jcr_content/hero/hero_banner/images/imageSlide8.img.620.high.jpg
Neural pruning is the process of synapse elimination, by retraction of the axons from unwanted synaptic connections.
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.
Apoptosis of neurons involves the killing of the cell and elimination of all connections associated with the neurons.
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.
Neuroplasticity is seen throughout the life of an organism, but brain show a much higher degree of plasticity during early childhood.
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.
Theories explaining memory and learning depend upon the phenomena of neuroplasticity.
A.1.A2 Events such as strokes may promote reorganization of brain function.
Brain injuries and neuroplasticityConditions 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.
https://d1gqps90bl2jsp.cloudfront.net/content/brain/134/5/1264/F1.large.jpg
*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.
Bibliography / Acknowledgments
Bob Smullen