AF - Mechanisms of circuit formation Flashcards
How many connections are in the brain?
10^14
What was Weiss’s hypothesis about the connectivity in adults?
- Random connections and diffuse neuronal outgrowth occurs to all targets
- Followed by elimination of non-functional connections
- Activity of the neuron reinforces connection
When did Weiss state his hypothesis?
1928
What was Sperry’s hypothesis about the connectivity in adults?
CHEMOAFFINITY HYPOTHESIS
- Directed and specific axon outgrowth occurs through axons, following “individual identification tags” carried by “cells and fibres” of the embryo
- Chemicals are followed by growth cones
When did Sperry state his hypothesis?
1939
What is the superior colliculus termed in the Xenopus?
The tectum
When an object is seen, what happens as the image passes through the lens?
It is flipped, so that on the retina, the image is upside-down and back to front
How is the upside-down image on the retina flipped the correct way again?
The connections from the retina are organised so that:
- The axons from the nasal side of the retina (more ANTERIOR) go to the POSTERIOR of the tectum
- The axons from the temporal side of the retina (more POSTERIOR) for the ANTERIOR of the tectum
- There is a graduation of axons, corresponding topological mapping of the retina in the tectum, forming a fine map
What was the experiment done by Sperry to show that his hypothesis was correct?
- Cut optic nerve and removed temporal part of the retina (so that ONLY nasal axons can grow back)
- The regrowing nasal axons grew through but ignored the territory of the temporal axons
- He also ablated the edges of both the nasal and temporal neurons retina, only leaving the central retina
- Neurons grew back
- Shows that the mapping of the retinal ganglion in the tectum was graded
What would happen to the nasal neurons in Sperry’s experiment, if Weiss was correct?
The nasal neurons would grow everywhere and then be ‘pruned’ back
What did Sperry’s experiments show and not show evidence for?
- Neuron growth during axon regeneration
- Not what happens in the developing embyro
In regards to axon outgrowth, what happens in the embryo?
- Patterns of axon outgrowth which is highly organised. reproducible and stereotyped
- In the mouse, drosophila, zebrafish and human
What experiment was done to test if a specific neuron knew where to grow?
What was the control for this experiment?
- Reverse part of the spinal cord between T7-LS3 and see if the axons still know where to grown
- The motor axons still knew where to find their way to their muscle targets
- Showing that axons navigate to their targets and different neurons know where to send their axons
Control: Taking out this part of the spinal cord and putting it back in the same way
What are guidance cues?
Factors in the environment of a neuron which axons use to find their correct targets
What is a growth cone?
The growing tip of the axon which senses guidance cues in the environment
Who proposed the growth cone?
Cajal
Why were insects used to identify the location of guidance cues?
- Relatively simple nervous system
- Easy to observe and manipulate
- Can ablate the cells using lasers (in grasshoppers)
What did Corey Goodman discover?
In the grasshopper, identified almost every neuron in the embryonic nerve cord and draw a map of axon projections
How are development pathways in embryos the same?
s
Where can guidance cues be found?
s
What happens when a growth cone encounters an axon?
s
What is the ‘labelled pathway hypothesis’?
s
Where are subplate neurons found and what neurons do they provide scaffold for?
How is this proved?
s
What do pioneer neurons supply for follower neurons and how?
s
How do the pioneer axons find their way to the target in an apparently ‘featureless’ environment?
s
What are the characteristics of pioneer axon pathways?
What do their growth cones do?
s
In the grasshopper limb, what happens to the Ti1 (pioneer) growth cone?
s
What happens in the grass hopper limb when Cx1 cell is ablated?
s
Where is Cx1 present in the grasshopper limb?
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Does Cx1 have any obvious morphological features?
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What happens when ‘stepping stones’ or ‘guidepost cells’ are ablated?
s
Where do the molecular differences in the environment come from, which guide pioneer axons to the correct place?
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What pathways do axon tracts in the forebrain do?
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What defines particular regions of the brain?
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What pathways do axon tracts in the hindbrain do?
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How is the hindbrain compartmentalised?
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What pathways do axon tracts in the spinal cord do?
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What are the 4 forces of axon guidance and describe them?
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Where are axon cues located?
s
in the grasshopper limb, what suggests that axon guidance cues can be attractive?
s
In the grasshopper limb, what suggests that axon guidance cues can be repulsive?
s
What type of guidance is used by guidepost cells and how?
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How does contact attaction/repulsion occur?
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