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Developing Nervous Systems > Growth and Guidance of Axons > Flashcards

Growth and Guidance of Axons Flashcards

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1
Q

How many genes are there in the human genome?

A

Around 20,000

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2
Q

What are the names of the two people who made theories on how specific neuronal connectivity arises?

A

Wiess - Resonance Theory

Sperry - Chemoaffinity Hypothesis

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3
Q

Describe Weiss’s Resonance Theory

A

Stochastic/ random and diffuse neuronal outgrowth first occurs towards all targets, and then their is the elimination of non-functional connections

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4
Q

Describe Sperry’s Chemoaffinity Theory

A

Directed and specific outgrowth occurs through axons following ‘individual identification tags’ carried by cells and fibres of the embryo
There is no pruning back

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5
Q

Describe the basic anatomy of the newts visual system

A

They have a lens -> Retina -> Tectum
The nasal retina is more anterior on the eye but projects most posteriorly on the the tectum
The temporal retina of the eye is posterior on the eye but projects anteriorly onto the tectum

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6
Q

How did Sperry test which theory was right?

A

He cut the optic nerve and removed the temporal retina - this allowed only the nasal axons to grow back
If Sperry was right the nasal axons would go back and only go to the posterior end of the tectum
If Weiss was right there would be lots of growth first and the axons would be pruned back later
He found that the axons grew back directly to the right place

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7
Q

What did Sperry see in his newt experiment?

A

He saw that patterns of outgrowth were highly organised, sterotyped and reproducible

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8
Q

How do motor neurons develop in the chick embryo?

A

They are guided specifically to their targets

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9
Q

How can it be shown that motor neurons are guided in the chick embryo?

A

Normally the LS3 neuron innervates T7
If LS3 is removed and put in a different place in the embryo the neuron still finds its way to the target even though the motor neurons are in the wrong place

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10
Q

What are guidance cues?

A

Factors in the enviroment that axons can use to fins their correct target

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11
Q

What did Cajal propose?

A

That the growing tip of the axon - the growth cone - sensed cues in the enviroment

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12
Q

What could be the result of ablating cells?

A

Ablating cells can change the axon pathway

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13
Q

What is the labelled pathway hypothesis?

A

Axons can selectively fasiculate with other axons
Axon surfaces carry labels or cues
Different growth cones express different receptors for cues

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14
Q

What is the role of early axons

A

Pioneer axons form an axon scaffold which later neurons can follow and extend on

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15
Q

What is the role of subplate neurons in mammals?

A

They project from the cortex to the thalamus prior to the innervation of the cortex by the lateral geniculate nucleus neurons

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16
Q

What happens if you ablate the subplate neurons

A

The geniculate neurons fail to innervate the thalamus showing that the subplate neurons are required as a scaffold

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17
Q

What is the general pathway of the Til axon in the grasshopper ie where does it turn?

A

It turns at the the limb boundary (down) and then again as it approaches the specific cell Cx1

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18
Q

Ablation of Cx1 in the grasshopper has what effect on the Til axon?

A

It stalls at the other side of the limb boundary and does not cross it

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19
Q

What are the four ways in which guidance cues can act?

A

1) Contact attractant
2) Contact repulsion
3) Chemoattraction
4) Chemorepulsion

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20
Q

Why are aplysia growth cones ideal when studying growth cones?

A

They have flattened growth cones so it is easy to see their components

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21
Q

What are the components of the growth cone?

A

Central part - contains microtubule
Transitional part
Peripheral part - contains actin

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22
Q

Lamella and filopodia are made up of what?

A

F-actin

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23
Q

What is the structure of actin in lamella?

A

Actin bundles are cross-linked into a net

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24
Q

What is the structure of actin in filopodia?

A

The actin bundles are polarised to form larger bundles

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25
Describe F actin treadmills in a resting growth cone
There is clear tracking of the actin from the periphery of the growth cone to the centre - the actin is recycled after being broken down into the actin subunits
26
What is the action of tubulin?
Tubulin is mainly centralised but will shoot up and down the back bone of the filopodia
27
What happens when a growth cone comes into contact with an attractive cue?
F-actin treadmilling slows and accumulates | This accumulation causes the filopodia to become stable and it drags microtubules to the back of the filopodia
28
What two key components lead to filopodial extension and reorganisation of microtubules?
The flow backwards is arrested by the engagement of a molecular clutch - receptor binds to cues which causes extension and results in the forward movement of the filopodia Also an action-myosin-based actin-tubulin link pulls microtubules into the wake of the extending filopodia
29
What is growth cone collapse?
A growth cone comes into contact with an axon and collapses A connection is maintained showing that a signal is being sent so the growth cones do not fall back due to the lack of adhesions F-actin is destabilised
30
What are semaphorins?
A family of inhibitory guidance cues They can be membrane bound or secreted Secreted semaphorins can cause neurons to collapse effecting the f-actin
31
What are the two substrates that axons can put down and what are they like?
Poly-ornithine (porn, sticky) | Pallodium (non sticky)
32
Which substrate do growth cones not grow over?
Pallodium
33
Axon growth is better on what?
Laminin despite the adhesion to collagen is better
34
What does laminin do in relation to growth of axons and how can this be shown?
It does not direct growth but it allows it This can be seen because blocking the receptor for laminin does not change the direction of growth but does slow it Gradients of laminin in vitro do not direct axon growth
35
Where is semaphorin 1 expressed?
Along the limb body boundary where Ti1 turns and avoids
36
What is the effect of using semaphorin 1 antibodies?
The Ti1 axon crosses the limb body boundary
37
What happens in mice lacking semaphorin 3A?
Axons will stray into the wrong territories of the limb
38
What are ephrins?
Non-permissive factors that cause contact repulsion They are detected by cell surface receptors called ephs They are tyrosine kinases They cause repulsion
39
What does commisural mean?
It crosses the midline
40
What is the normal pathway of sensory relay neurons in the rodent spinal cord?
They are commisural, they grow from the roof plate towards the floor plate
41
What experiment did Placzek and Tessier Lavigne perform?
They took dorsal explants = normal growth in the culture initially The axon will turn towards the floor plate as the floor plate secretes a chemoattractant They purified the floor plate to find the chemoattractant netrin
42
What is netrin?
Netrin is secreted and is similar to laminin, which can associate within the extracellular matrix Cells expressing netrin attract axons The effect of netrin is only short
43
Cells expressing BMP repel or attract axons?
Repel
44
What does cyclpopamine block?
Shh signalling
45
What is cre recombinase?
It allows bacteriophage p1 to insert its DNA into the bacteria hosts genome Its own DNA can then replicate
46
What does cre bind to?
A specific 34 base pair sequence called loxP which it can cut and rejoin to another loxP site
47
What is cre recombinase and loxP used for?
To delete DNA specifically between loxP sites - the removed gene is degraded
48
Gradients of morphogens are re used later for what?
To shape axons | Shh and BMPs specify neural plate
49
What happens if you knockout or lower the gradient of semaphorin 2?
Ti1 guidance is disrupted in a way that suggests that secreted sema2 directs Ti1 growth cones towards the body
50
When do growth cones re-programme?
When intermediate or choice points are encountered
51
When do commisural axons lose responsiveness to netrins?
After they cross the midline | This explains why in the hindbrain commisural axons are able to continue past the floor plate without turning
52
What are lipophilic dyes used for?
Image tracing, membrane tracking and axon tracing
53
When do commisural axons become sensitive to repellents?
After they cross the midline
54
What are the two key genes involved in preventing a an axon to recross the midline
Roundabout and commisureless
55
What are slit proteins?
Inhibitory proteins found in the floor plate
56
What does the roundabout gene code for?
A slit protein
57
Where is roundabout expressed?
It is expressed in high levels on axons that do not cross the midline Commisural axons initially express low levels but they express high levels after they have crossed the midline
58
What is the result of a mutation in roundabout?
The axons cross the midline multiple times
59
Where is commisural expressed?
In axons that cross the midline - it is switched off after they have crossed
60
What happens if the commisural protein is missing?
Roundabout is expressed at much higher levels in the cells that would normally cross the midline and so the axons extend longitudinally so they do not cross the midline
61
How does commisural control roundabout?
At a post transcriptional level locally Como encodes a trafficking protein which prevents robo from reaching the cell surface so that the growth cone can not receive slit inhibitory signals
62
What is the commisural homologue in vertebrates?
There is not one | However, there is another robo like gene called Rig1 which is expressed only in pre crossing fibres to block robo1
63
What is the roundabout homologue in vertebrates
Robo1
64
What is the role of fascicilin ii in insects?
It binds one cell surface to another cell surface If expressed in cells that do not normally adhere, fascii can cause adhesion It controls fasciculation in flies It also controls defasciculation so that the right motor neuron can leave to innervate muscle
65
What is a discrete target?
A single cellular target
66
What does ablating specific muscle precursor cells show about the development of motor neurons?
The motor neurons fail to leave the main motor trunk at the appropriate branch points which suggests they are looking for specific labels
67
What occurs in topographic maps?
Neighbouring neurons send axons to neighbouring sites in their targets to maintain the topology in the target as in in the retinotectal system
68
What does the stripe assay show?
That cells from the posterior tectum make a non-permissive factor that repels temporal retinal axons Nasal axons ignore whether they are in a posterior or anterior part but temporal axons avoid growing on posterior stripes so they must be avoiding a repellent factor
69
What happens if the posterior stripes are treated and what does this show?
The temporal axons will now grow on the posterior strips showing that a repellent protein must have been denatured
70
What are the inhibitory proteins in the posterior tectum?
Two ephrins - ephrin A2 and ephrin A5
71
What happens in mice if ephrin A2b and A5 is knocked out in mice?
The temporal axon grows to the posterior tectum so the topographic map is disordered

Developing Nervous Systems (11 decks)

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