Development of the nervous system Flashcards
What does the formation of the nervous system begin with?
The zygote (fertilised egg)
What is a morula?
A solid ball of cells (16 cells). Formed on day 4 after the zygote has begun to divide
Describe blastulation on day 5
Cells separate into two groups
1) inner cell mass (embryo)
2) Encompassing sac known as trophoblast (placenta) - these cells secrete fluid creating a cavity
Forms a blastocyst
What are the two layers formed by the cells of the inner cell mass?
Epiblast - goes onto form most of the embryo
Hypoblast - becomes part of the gut
What happens during day 13-19?
Gastrulation, formation of a groove known as the primitive streak
What is the end result that forms a gastrula through gastrulation?
A three layer embryo, consisting of the layers ectoderm, mesoderm and endoderm.
Which layer of the three layer embryo does the nervous system develop from?
Ectoderm
What happens in front of the primitive streak after 3 layer embryo is formed (gastrula)?
The endotherm thickens to become the neuroectoderm, which forms the neural plate
What happens after the neuroectoderm is made?
Uneven rates of cell-division - forms a groove known as the ‘neural groove’.
What does the neural groove become?
The midline of the embryo
What can be found on either side of the midline of the embryo?
Ridges of the ectoderm bulge
What happens during day 22?
Tops eventually come together to form the neural tube. The interior becomes the fluid filled ventricles of the brain and central canal of the spinal cord.
Forms neurula - neurulation
What happens on day 24?
Three major divisions of the brain are discernible.
Prosencephalon (telencephalon and diencephalon)
Mesencephalon
Rhombencephalon (metencephalon and myelencephalon)
+ neural tube
How does a tube become a brain?
Channel between spinal cord and brain partially closes
Fluid is pumped in to swell the brain
Differential rates of cell growth determine regions of enlargement
Regions of swelling demarcate future brain regions
What happens 3-4 weeks after the formation of neural tube?
The tube doesn’t remain in a straight line
What is the name of the tube that grows over the rest of the brain?
The telencephalic tube which forms the cerebral cortex
What happens as the telencephalic tube travels?
It forms the occipital and temporal lobes (and takes the fluid filled core with it)
What happens by 6 months(fetus)?
Early stage sulci and gyri of the cerebral cortex
Where do neurons come from?
During gastrulation, cells in the ectoderm around midline receive a chemical signal (e.g noggin)
From the notochord
Causes some of them to develop into neural precursor cells
Neural stem cells - divide (hall-mark of stem cells)
This stage is called Proliferation - symmetrical division
What happens following neurulation?
Following neurulation, as the pre-cursors divide, they produce a primitive neuron (neuroblast) or glial cell (glioblast) and another precursor (asymmetrical division)
What happens after the pre-cursors divide and produce either a neuroblast, glial cell, or precursor?
Migration - most neurons/glial cells are produced in the tube’s ventricular zone. Neuroblasts/glioblasts migrate to their final destination with the help of guidance mechanisms
What happens when neuroblasts/glioblasts reach their target?
They need to decide what type of neuron or glial cell they want to be - determined by local chemical interactions with neighbouring cells.
Then interconnect to form neuronal circuits
What do immature neurons produce?
A small number of extensions - neurites. Eventually commits to one of these being the axon and the others dendrite
What do axons do specialised terminals for?
Known as growth cones - to find their targets
What do growth cones consist of?
Lamellipodia (sheet like expansion) and filopodia (fine processes)
What do receptors in filopodia do?
Sense attractant or repellent chemical cues in the evironment
How do axonal growth cones navigate?
Using diffusible agents (chemoattractants or chemorepellents)
And non-diffusible agents
- contact attractants and contact repellents
How does the volume of the brain quadruple between birth and adulthood?
Through synaptogenesis, myelination, and dendritic branching
What is post-natal hydrocephalus?
Build-up of cerebrospinal fluid within the brain
Why are postnatal changes possible?
The brain is still being wired up after birth, and postnatal wiring of the brain can be influenced by experience
What is the main visual pathway in humans?
The geniculostriate system
What does the left and right primary visual cortex receive and why?
Information from both of the eyes
- the temporal retina projects to the same side of the brain
- the nasal retina projects to the other side.
So both eyes are used
What happens in carnivores and apes and their primary visual cortex?
Inputs carrying information from the two eyes form an alternating series of eye-specific domains in the primary visual cortex (ocular dominance columns)
What is the critical period?
Needing stimulation in order to allow the system to get wired up.
What is the critical period for vision in cats?
First three months
What does monocular deprivation affect?
The width of ocular dominance in the visual cortex
Describe the association between columns and deprived/nondeprived eyes?
Columns related to the non-deprived eye are wider than normal, those related to the deprived eye are shrunken
What is the evidence of neuroplasticity occurring in adults?
London taxi drivers have bigger posterior hippocampi
The visual cortex responds to braille in late-blind individuals
N = 3 subjects
Complete blindness post-puberty due to glaucoma and/or cataract
PET using h2015 – radiolabelled water (injected into these people)
Reading braille leads to activity in the primary visual cortex
What are the mechanisms that allow the brain to be so plastic?
Axonal sprouting and adult neurogenesis
What is axonal sprouting
———-??
What are the two areas that neurons and glial cells regenerate in during adult neurogenesis?
The olfactory bulb and the hippocampus
Describe the olfactory bulb and it’s role in adult neurogenesis?
the area adjacent to the lateral ventricles provide the neuroblasts that later form neurons in the olfactory bulb
Describe the hippocampus in adult neurogenesis
In the hippocampus, produced in area of the dentate gyrus called the sub-granular zone
Become integrated into synaptic circuits
