A1 - Neural development

What is embryogenesis

The development of a fully-formed organism from a fertilised egg

Where are all tissues derived from?

three initial germ layers (ectoderm, mesoderm, endoderm) formed via gastrulation

What is neurulation

The process by which a neural tube in embryonic chordates is formed

Neurulation process

• Cells located in the outer germ layer (ectoderm) differentiate to form a neural plate

• The neural plate then bends dorsally (towards the back), folding inwards to form a groove with a neural crest on either side

• 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

Xenopus and their use

genus of frog that possess robust embryos that can tolerate extensive manipulation

• therefore they can be used as a suitable animal model for investigating the developmental stages of embryogenesis

Neurulation in a Xenopus Embryo

which areas does closure of the neural tube occur more slowly and faster in the embryo?

• Closure of the neural tube occurs more slowly in the caudal (tail) region of the embryo,

• while the area where the brain forms is well advanced in closure.

Spina bifida + effects

a birth defect resulting in the incomplete closure of the neural tube (and associated vertebrae)

• The vertebral processes do not fuse, leaving the spinal cord nerves exposed and prone to damage

Normal vs spina bifida vertebrae comparison

Spina bifida occulta

the splits in the vertebrae are so small that the spinal cord does not protrude (less severe/mild)

Spina bifida cystica

a meningeal cyst forms (meningocele) which may include the spinal elements (myelomeningocele) (less severe/mild)

Severe spina bifida - what happens

patients may typically suffer some degree of paralysis, as well as bowel and bladder dysfunction

Believed cause of spina bifida

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 (folic acid)in the diet during pregnancy is believed to play a significant role in causing spina bifida

What does the neural tube contain

multipotent neuronal stem cells which can differentiate to form the different types of nerve cells:

• neurons

• glial cells

Neurons

are specialised nerve cells that conduct messages:

• they can be sensory,

• motor

• or relay (interneurons)

Glial cells

provide physical and nutritional support for the neurons

• roughly 90% of nerve cells in the brain are glial cells

How are neurons produced?

Via neurogenesis by progenitor neuroblasts

Do neurons regenerate?

Not usually.

• Because most neurons survive for the lifetime of the individual and do not reproduce following embryogenesis

• Certain brain regions may be capable of adult neurogenesis, but most of the nervous system is incapable of regeneration

Neuron production via neurogenesis

What must immature neurons do and why?

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

2 processes via which neural migration may occur?

glial guidance or somal translocation

glial guidance

Glial cells may provide a scaffolding network along which an immature neuron can be directed to its final location

somal translocation

the neuron may form an extension at the cell’s perimeter and then translocate its soma along this length

what does an immature neuron consist of

cell body (soma) containing a nucleus and cytoplasm

when will axons and dendrites grow from an immature neuron

• 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)

what are filiopida

a growth cones at the tip of axons that contain highly motile growth filaments

extension of filiopida results

Extension of these filopodia causes the expansion of the internal cytoskeleton within the growth cone

• this results in growth

what is the extension of filiopida controlled by - what does this allow the cells to do

chemical stimuli released from surrounding cells

• These cells may release chemoattractant signals (grow towards) or chemorepellant signals (grow away)

• Using these molecular guidance signals, axon growth cones may navigate long distances to reach specific targets

what is a synapse

a junction at which a neuron transmits a signal to another cell

• Most synapses transmit chemical signals, although electrical synapses also exist

what will a developing neuron form… what will this allow

multiple synapses, creating a vast array of permutable communication pathways

what will a neuron form and with whawt within the central nervous system

Within the CNS, a neuron may form a synapse with another axon, dendrite or cell body (soma)

what will a neuron form within the peripheral nervous system

Within the PNS, a neuron may form a synapse with a muscle fibre (neuromuscular) or gland (neuroglandular)

neurosecretion

Some neurons may form a synapse with capillaries and secrete chemicals directly into the bloodstream (neurosecretory)

What is the role of synapses in the process of learning? how do neurons evolve / strengthen and weaken

• During embryonic and early post-natal development, neurons form multiple synapses to maximize available connections.

• As an organism matures, some synapses are used more frequently, strengthening those connections.

• Conversely, synapses that are not used as often weaken and do not persist.

• This process of strengthening and weakening certain neural pathways is essential for learning.

Neural pruning and how it works

Neural pruning involves the loss of unused neurons (by removing excess axons and eliminating their synaptic connections)

Infant vs adult brains no. of neurons and no. of synaptic connections

• Infant and adult brains typically have the same total number of neurons (roughly 100 billion neurons in total)

• However infant brains form vastly more synaptic connections (approximately twice the number found in adult brains)

purpose of neural pruning

to reinforce complex wiring patterns associated with learned behaviour

what influences neural pruning

• Pruning is influenced by environmental factors and is

• mediated by the release of chemical signals from glial cells

Synaptic Formation and Neural Pruning

Neuroplasticity

the capacity for the nervous system to change and rewire its synaptic connections

what does neuroplasticity enable

• enables individuals to reinforce certain connections (learning) or avoid damaged regions

• enables memory retention and learning

by which 2 mechanisms is neuroplasticity achieved

via two primary mechanisms

• rerouting

• sprouting

Rerouting

creating an alternative neural pathway by deleting damaged neurons and forming a new pathway between active neurons.

Sprouting

involves the growth of new axon or dendrite fibres to enable new neural connections to be formed

what is a stroke

is the sudden death of brain cells in a localised area due to inadequate blood flow

what do strokes result in

This results in the improper functioning of the brain, due to the loss of neural connections in the affected area

two main types of strokes

• ischemic strokes

• hemorrhagic strokes

ischemic strokes

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

how may stroke symptoms be temporary and what is this a result of

• 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

types of strokes diagram