AF - Mechanisms of circuit formation

Question 1

How many connections are in the brain?

Answer 1

10^14

Question 2

What was Weiss’s hypothesis about the connectivity in adults?

Answer 2

• Random connections and diffuse neuronal outgrowth occurs to all targets
• Followed by elimination of non-functional connections
• Activity of the neuron reinforces connection

Question 3

When did Weiss state his hypothesis?

Answer 3

1928

Question 4

What was Sperry’s hypothesis about the connectivity in adults?

Answer 4

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

Question 5

When did Sperry state his hypothesis?

Answer 5

1939

Question 6

What is the superior colliculus termed in the Xenopus?

Answer 6

The tectum

Question 7

When an object is seen, what happens as the image passes through the lens?

Answer 7

It is flipped, so that on the retina, the image is upside-down and back to front

Question 8

How is the upside-down image on the retina flipped the correct way again?

Answer 8

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

Question 9

What was the experiment done by Sperry to show that his hypothesis was correct?

Answer 9

• 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

Question 10

What would happen to the nasal neurons in Sperry’s experiment, if Weiss was correct?

Answer 10

The nasal neurons would grow everywhere and then be ‘pruned’ back

Question 11

What did Sperry’s experiments show and not show evidence for?

Answer 11

• Neuron growth during axon regeneration

- Not what happens in the developing embyro

Question 12

In regards to axon outgrowth, what happens in the embryo?

Answer 12

• Patterns of axon outgrowth which is highly organised. reproducible and stereotyped
• In the mouse, drosophila, zebrafish and human

Question 13

What experiment was done to test if a specific neuron knew where to grow?

What was the control for this experiment?

Answer 13

• 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

Question 14

What are guidance cues?

Answer 14

Factors in the environment of a neuron which axons use to find their correct targets

Question 15

What is a growth cone?

Answer 15

The growing tip of the axon which senses guidance cues in the environment

Question 16

Who proposed the growth cone?

Answer 16

Cajal

Question 17

Why were insects used to identify the location of guidance cues?

Answer 17

• Relatively simple nervous system
• Easy to observe and manipulate
• Can ablate the cells using lasers (in grasshoppers)

Question 18

What did Corey Goodman discover?

Answer 18

In the grasshopper, identified almost every neuron in the embryonic nerve cord and draw a map of axon projections

Question 19

How are development pathways in embryos the same?

Answer 19

s

Question 20

Where can guidance cues be found?

Answer 20

s

Question 21

What happens when a growth cone encounters an axon?

Answer 21

s

Question 22

What is the ‘labelled pathway hypothesis’?

Answer 22

s

Question 23

Where are subplate neurons found and what neurons do they provide scaffold for?

How is this proved?

Answer 23

s

Question 24

What do pioneer neurons supply for follower neurons and how?

Answer 24

s

Question 25

How do the pioneer axons find their way to the target in an apparently ‘featureless’ environment?

Answer 25

s

Question 26

What are the characteristics of pioneer axon pathways?

What do their growth cones do?

Answer 26

s

Question 27

In the grasshopper limb, what happens to the Ti1 (pioneer) growth cone?

Answer 27

s

Question 28

What happens in the grass hopper limb when Cx1 cell is ablated?

Answer 28

s

Question 29

Where is Cx1 present in the grasshopper limb?

Answer 29

s

Question 30

Does Cx1 have any obvious morphological features?

Answer 30

s

Question 31

What happens when ‘stepping stones’ or ‘guidepost cells’ are ablated?

Answer 31

s

Question 32

Where do the molecular differences in the environment come from, which guide pioneer axons to the correct place?

Answer 32

s

Question 33

What pathways do axon tracts in the forebrain do?

Answer 33

s

Question 34

What defines particular regions of the brain?

Answer 34

s

Question 35

What pathways do axon tracts in the hindbrain do?

Answer 35

s

Question 36

How is the hindbrain compartmentalised?

Answer 36

s

Question 37

What pathways do axon tracts in the spinal cord do?

Answer 37

s

Question 38

What are the 4 forces of axon guidance and describe them?

Answer 38

s

Question 39

Where are axon cues located?

Answer 39

s

Question 40

in the grasshopper limb, what suggests that axon guidance cues can be attractive?

Answer 40

s

Question 41

In the grasshopper limb, what suggests that axon guidance cues can be repulsive?

Answer 41

s

Question 42

What type of guidance is used by guidepost cells and how?

Answer 42

s

Question 43

How does contact attaction/repulsion occur?

Answer 43

s