L10 - Axon Gudiance

Question 1

How many neurons does the human brain consist of?

Answer 1

Human brain consists of 1011 neurones

Each making >103 connections = 1014 connections

Question 2

How does specific neuronal connectivity arise? - Weiss

Answer 2

Resonance theory – 1928

• Stochastic and diffuse neuronal outgrowth
• To all targets followed by elimination of non-functional connections

Question 3

How does specific neuronal connectivity arise? - Sperry

Answer 3

Chemoaffinity hypothesis – 1939

• Directed and specific outgrowth
• Through axons following individual identification tags carried by cells/fibres of the embryo

Question 4

Sperrys experiment

Answer 4

Cut optic nerve and remove temporal retina
- Allows just nasal axons to grow back
The regrowing axons grew though ignoring the territory normally innervated by the axons
- They grew back directly to the right place

Question 5

Axon pathways

Answer 5

If Weiss were right - random patterns of axons in embryos

In reality the patterns of axon outgrowth are highly organised, reproducible and stereotyped

Question 6

Experiment to test how axons are guided specifically to their targets in the chick

Answer 6

Cut and replace or reverse segment of neural tube before motor axons grow out
In mutant
- Despite displacement of cell bodies T7 and LS1 motor axons still reach normal muscle target
In control
- Normal innervation of specific limb muscles by motor axons from T7 and LS1

Shows axons can navigate to their targets

Question 7

What are guidance cues?

Answer 7

Environment contains factors that axons use to find their targets

Question 8

What we need to know about guidance cues?

Answer 8

Nature of guidance cues and where they are located
How axons detect and respond to them
What strategies are used so a limited number of cues can make a complex nervous system

Question 9

What is Caias growth cone?

Answer 9

Growing tip of the axon sensed cues in the environment

Question 10

Experiments to identify locations of guidance cues

Answer 10

Were done in insects

• Simple nervous system
• Embryos easy to observe and manipulate
• In larger insects individual cells ablated using laser

Question 11

Grasshopper - map of axon projections

Answer 11

Detailed analysis resulted in identification of almost every neurone in the embryonic nerve cord - map of axon projections

Question 12

Cues can be found on axons surface

Answer 12

Reproducibility of axon behavior suggested growth cones were responding to cues in environment
Pathways changed when specific axons were encountered

Question 13

Tested idea that cues can be found on axons surface

Answer 13

Tested by ablating cells that might carry potential cues
G axon stalls in absence of P axon
- Not due to lack of axons on which to extend
- Not due to reduction in numbers of axons
- Because G axon growth cone looking for specific cues on P axon

Question 14

Labelled pathway hypothesis

Answer 14

Axons
- Can selectively fasciculate with other axons
- Carry labels/cues
Different axon growth cones express different receptors for the cues

Question 15

Axon scaffolds

Answer 15

Early axons (pioneers) form an axon scaffold on which later axons (followers) can extend

Question 16

Axon scaffolds are important in vertebrates

Answer 16

Subplate neurons project from cortex to thalamus prior to innervation of cortex by lateral geniculate nucleus neurons
LGN innervation fails in this region

Question 17

Ablating part of subplate early on before axons extend leads to?

Answer 17

LGN innervation fails in this region

Question 18

First axon navigation

Answer 18

First axons navigate in an apparently featureless environment

Question 19

Pioneers follow stereotyped paths

Answer 19

Pioneer axon growth cones appear to react at specific points in the pathway

Question 20

Pioneers in Grasshopper limb

Answer 20

Pioneer Ti1 growth cone makes a specific turn
- At limb boundary
- As it approaches a specific cell (Cx1)
Ablation of Cx1 causes the Ti1 growth cone to stall at the other side of the limb boundary
Neither Cx1 or limb boundary cells are distinguished by morphological features
Other cells when ablated also cause stalling - guidepost cells
Implies there must be molecular differences in the environment

Question 21

How to predict where axon tracts will form?

Answer 21

Use patterning information from the early embryo

Axon guidance cues are located on other axons and in many cell types in the early embryo

Question 22

Forebrain axon tracts

Answer 22

Axons follow boundaries of domains of patterning gene expression

Question 23

Spinal cord axon tracts

Answer 23

Axons are attracted to and follow boundaries of the floor plate

Question 24

Hindbrain axon tracts

Answer 24

Axons follow boundaries of rhombomeres

Question 25

Forces of axon guidance

Answer 25

Axon guidance cues may be both positive and negative

• Cell ablations lead to growth cones stalls - as if an attractive force has been lost
• Ti1 growth cone avoids the limb boundary - as if it were an inhibitory force

Question 26

What are the 4 ways in which guidance cues act?

Answer 26

Contact attraction
Contact repulsion
Chemoattraction
Chemorepulsion