A1 Neural Development Flashcards
Neural development
refers to the processes that generate, shape and reshape the nervous system
Embryogenesis
The development of a fully-formed organism from a fertilised egg
All tissues are derived from three initial germ layers (ectoderm, mesoderm, endoderm) formed via gastrulation
In chordates, a flexible notochord will develop during gastrulation and lead to the subsequent formation of a neural tube
Neurulation
The formation of a neural tube in embryonic chordates
Cells located in the outer germ layer (ectoderm) differentiate to form a neural plate
The neural plate then bends dorsally, folding inwards to form a groove flanked by a neural crest
The infolded groove closes off and separates from the neural crest to form the neural tube
The neural tube will elongate as the embryo develops and form the central nervous system (brain and spinal cord)
The cells of the neural crest will differentiate to form the components of the peripheral nervous system
Neurulation in a Xenopus
Xenopus are a genus of frog that possess robust embryos that can tolerate extensive manipulation
This makes them a suitable animal models for investigating the developmental stages of embryogenesis
During neurulation, the following embryonic tissues should be easily identifiable:
Three germ layers (outer = ectoderm ; middle = mesoderm ; inner = endoderm)
A hollow cavity called the archenteron (will develop into the digestive tract)
Notochord (flexible rod that stimulates neurulation)
Neural tube (developed from the infolding of the neural plate)
Spina Bifida
is caused by a malformation of the caudal portion of the neural tube
It results in the incomplete closure of the neural tube (and associated vertebrae)
The severity of the condition can vary from mild to severe depending on the consequence of the incomplete closure
In cases of spina bifida occulta, the splits in the vertebrae are so small that the spinal cord does not protrude
In spina bifida cystica, a meningeal cyst forms (meningocele) which may include the spinal elements (myelomeningocele)
In the more severe cases, patients may typically suffer some degree of paralysis, as well as bowel and bladder dysfunction
Spina bifida is believed to be caused by a combination of genetic and environmental factors
The average worldwide incidence of the condition is ~1 in 1,000 births, however marked geographic variation occurs
Not having enough folate in the diet during pregnancy is believed to play a significant role in causing spina bifida
Neurons
are specialised nerve cells that conduct messages – they can be sensory, motor or relay (interneurons)
they are produced by progenitor neuroblasts via a process known as neurogenesis
Glial cells
provide physical and nutritional support for the neurons – roughly 90% of nerve cells in the brain are glial cells
Neural migration
Immature neurons must migrate in order to adopt precise final positions that allow for the formation of neural circuitries
This migration process is critical for the development of brain and spinal architecture
Neural migration may occur via one of two distinct processes – glial guidance or somal translocation
Glial cells may provide a scaffolding network along which an immature neuron can be directed to its final location
Alternatively, the neuron may form an extension at the cell’s perimeter and then translocate its soma along this length
Axon and dendrite formation
An immature neuron consists of a cell body (soma) containing a nucleus and cytoplasm
Axons and dendrites will grow from each immature neuron in response to chemical signals from surrounding cells
Some axons may be quite short (within the CNS) but others may extend to other parts of the body (within the PNS)
Development of Neurons (summary)
- cell division in the neural tube produces large numbers of cells that differentiate into neurons
- axons grow out of immature neurons in response to chemical stimuli
- some axons extend beyond the neural tube to reach other parts of the body
- some immature neurons migrate to their final location and become sensory or motor neurons
- developing neurons form multiple synapses with other neurons
- it is a use it or lose it approach (if the synapses are not being used then neural pruning occurs)
Synapses + neurons
A synapse is a junction at which a neuron transmits a signal to another cell (relay neuron or effector)
Most synapses transmit chemical signals, although electrical synapses also exist
A developing neuron will form multiple synapses, creating a vast array of permutable communication pathways
Within the CNS, a neuron may form a synapse with another axon, dendrite or cell body (soma)
Within the PNS, a neuron may form a synapse with a muscle fibre (neuromuscular) or gland (neuroglandular)
Some neurons may form a synapse with capillaries and secrete chemicals directly into the bloodstream (neurosecretory)
Neural pruning
involves the loss of unused neurons (by removing excess axons and eliminating their synaptic connections)
The purpose of neural pruning seems to be to reinforce complex wiring patterns associated with learned behaviour
Pruning is influenced by environmental factors and is mediated by the release of chemical signals from glial cells
Neuroplasticity
describes the capacity for the nervous system to change and rewire its synaptic connections
Neuroplasticity enables individuals to reinforce certain connections (learning) or circumvent damaged regions
Strokes + affect on brain
is the sudden death of brain cells in a localised area due to inadequate blood flow
There are two main types of stroke:
- Ischemic strokes result from a clot within the blood restricting oxygenation to an associated region of the brain
- Hemorrhagic strokes result from a ruptured blood vessel causing bleeding within a section of the brain
Strokes symptoms may be temporary if the brain is able to reorganise its neural architecture to restore function
Following a stroke, healthy areas of the brain may adopt the functionality of damaged regions
This capacity for the restoration of normal function is made possible due to the neuroplasticity of the brain
