L10-12 Axonal Guidance

Question 1

How many connections are there estimated to be in the human brain?

Answer 1

10^14

Question 2

How many neurons are there estimated to be in the human brain?
How many connections do they each make ?

Answer 2

10^11 each make 1000 connections

Question 3

What was Weiss’s theory?

Answer 3

Resonance theory: Random and diffuse neuronal outgrowth occurs to all targets - followed by elimination of non-functional connections.

Question 4

What was Sperry’s hypothesis?

Answer 4

Chemoaffinity hypothesis - directed and specific outgrowth through axons following individual identification tages

Question 5

Describe the passage of light from an object through the anatomy of the visual system to the tectum/superior colliculus

Answer 5

Light from the object is reversed in orientation during passage through the lens. This is then reversed again by the mapping of retinal ganglion axons to the neurons of the tectum/SC: nasal (ant.) goes to posteior; temporal to anterior; dorsal to ventral; ventral to dorsal.

Question 6

Where do nasal retinal ganglion neurones project to?

Answer 6

Posterior of the tectum

Question 7

Where do temporal retinal ganglion neurones project to?

Answer 7

Anterior of the tectum

Question 8

Describe what would happen if the optic nerve and temporal retina were ablated, had WEISS been correct

Answer 8

Nasal axons would grow back throughout the entire AP axis, but innervation of the anterior tectum would then be eliminated, while posterior connections survived

Question 9

Describe what actually happens if the optic nerve was cut and the temporal retina was ablated in adult newts or frogs. Whose hypothesis does this support?

Answer 9

Axons would grow back in exactly the right place. Importantly, nasal axons would grow through the anterior ignoring the territory normally occupied by the temporal axons, even though they are not present.
Supports Sperry

Question 10

Is the ‘optic nerve cut and temporal retina ablation’ experiment enough evidence to show what happens during embryogenesis

Answer 10

No - this is during regeneration in the adult and not during development of the embryo

Question 11

Describe an experiment that demonstrates the guidance of motor axons to their targets

Answer 11

Remove section of spinal cord, reverse it and replace. Observe that axons regrowing from displaced positions still find their correct targets.

Question 12

What would the control experiment for the removal and reversal of the spinal cord be?

Answer 12

Cut out section and just put back in normally

Question 13

What were the results of the spinal cord reversal experiment done in chick?

Answer 13

Normal innervation - the motor neurones still manage to find their targets - suggests that axons are able to navigate to theeir targets

Question 14

What do we call the factors suggested to be present by the spinal cord removal - reversal experiment ?

Answer 14

Guidance cues

Question 15

Experiments in which specific axons were ablated with a laser in the grasshopper embryo formed the basis for the ____???_____ hypothesis.

Answer 15

Labelled pathway

Question 16

How can the labelled pathway hypothesis be tested

Answer 16

By ablation of cells whose axons are thought to carry labels and seeing the effect

Question 17

Describe the key tenets (points) of the labelled pathway hypothesis

Answer 17

Axons can selectively fasciculate with other axons
Axon surfaces carry guidance cues
Different growth cones express different sets of receptors for cues
Early axons (pioneers) form an axon scaffold on which later axons can extend

Question 18

What is an example of a pioneer neurone and its target

Answer 18

Ti 1 to Cx1 in the grasshopper limb
or
subplate neurons and the mammalian thalamus

Question 19

What was the name of Weiss’s theory for the establishment of axon connectivity ?
Describe this theory

Answer 19

Resonance Theory

Random and diffuse neuronal outgrowth occurs to all targets followed by elimination of non functional connections

Question 20

What was Sperry’s theory for axon guidance

Describe this theory

Answer 20

Chemoaffinity hypothesis
Directed and specific outgrowth through axons following individual identificaiton tags carried by cells and fibres of the embryo

Question 21

What is the mamalian equiv of the tectum

Answer 21

Superior colliculus

Question 22

Where in the tectum do nasal retinal axons project?

Answer 22

Posterior

Question 23

Where in the tectum do temporal retinal axons project?

Answer 23

Anterior

Question 24

What model system did Sperry first use - what did he move to

Answer 24

Newts –> Frogs

Question 25

When the optic nerve was cut and temporal retina was ablated what would be observed if …

WEISS WAS RIGHT

SPERRY WAS RIGHT

WHAT ACTUALLY HAPPENED

Answer 25

Weiss –> Nasal axons would grow back innervating the entire AP extent of the tectum - but only those projecting to the posterior would survive later elimination
Sperry –> The nasal axons would grow directly to the posterior tectum and innervate

Regrowing nasal axons grew back to the posterior tectum; IMPORTANTLY they grew through the anterior territory without innervating

Question 26

Is Sperry’s 1963 experiment - the severing of the optic nerve and temporal retina ablation - sufficient proof that axons are guided during development?

Answer 26

No – this is regeneration in the adult organism - no explanation for what occurs during development of the embryo.

For extra marks: in the regeneration experiment, debris from the cut nerve (e.g. remnants of axons) could be guiding, whereas these would not be present in the embryo

Question 27

What was the control in the experiment where T7 - LS3 was cut out and reversed

What were the results of this experiment

What conclusions could be made

Answer 27

Cut out but put back in the same orientation

Found that axons navigated back to their normal muscle targets

Environment must contain guidance cues

Question 28

What is Cajal’s ‘growth cone’ ?

Answer 28

Growing tip of the axon - growth cone - senses environmental cues

Question 29

Describe experimental evidence suggesting that cues may be located on axons?

What are the important experimental controls that support this idea?

Answer 29

In two populations of axons where axon 1 usually turns and fasciculates on axon 2:

Without 2 (ie when the cell body of 2 has been ablated before axon growth), 1 stalls at the normal turning point.

Controls:
a) there are other axons (e.g. 3) on which 1 could fasciculate (but doesn’t even when 2 missing).

b) Axon 1 grows normally when axon 3 is ablated instead of axon 2.

Question 30

Why may an axon stall when growth cues are located on the axons

Answer 30

NOT due to lack of axons on which to extend
NOT due to reduction in the number of axons
But due to the fact that one axon is ‘looking’ for specific cues from the other axons

Question 31

The idea that one axon is looking for cues on the surface of another is an example of

Answer 31

The labelled pathway hypothesis

Question 32

Describe the basic characteristics of the labelled pathway hypothesis

Answer 32

Axons can selectively fasiculate with other axons
Axon surfaces can carry labels/cues
Different axon gorwth cones express a different set of receptors for such cues
Early axons (pioneers) form an axon scaffold on which axons can extend
Establishes axon surfaces as one potential source of guidance cues
Axon scaffolds are important in vert. as well

Question 33

Describe the role the subplate plays in the guidance of thalamic LGN axons. What experimental evidence supports this? What does LGN stand for?

Answer 33

Subplate neurones project from the cortex to the thalamus prior to innervation of the cortex by the LGN
If part of the subplate is ablated before the axons extend LGN innervation fails to reach the region of the cortex in which the subplate was ablated.
LGN = lateral geniculate nucleus

Question 34

What must the first axons navigate in

Answer 34

An apparently featureless environment

Question 35

Describe the turns made by Ti1 as it approaches its target _____

Answer 35

Specific turn at the limb boundary and then again at the target cell : Cx1

Question 36

What is the effect of ablation of Cx1 on the path of Ti1 thorough the limb bud

Answer 36

Causes the growth cone to stall at the point it normally crosses the limb/body boundary

Question 37

What is thought to affect the pathway of Ti1 on its way to Cx1?

Answer 37

Stepping stone cells/ guide post cells and the limb/body boundary, where the molecular environments are different

Question 38

Give some examples of where the guidance cues are not located on other axons and the effect this has on the path of the axon

Answer 38

E.g.
Axons following the boundaries of rhombomeres
Boundaries formed by the domains of gene expression
Attraction to and following the boundaries of the floor plate `

Question 39

What are the four fources of axon guidance

Answer 39

Contact attraction or repulsion

Chemo-attraction or -repulsion

Question 40

Why was ablation of the temporal retina, as well as cutting the optic nerve, important in Sperry’s 1963 experiment?

Answer 40

Because this gave the remaining axons, those from neurons in the nasal retina, the choice of innervating either anterior or posterior tectum without having to compete with temporal axons.

Question 41

What explanations could there be for the turning of Ti1 at the limb/body boundary?

What experiments have been done that might distinguish between these possibilities?

Answer 41

The limb/body boundary (LBB) could be a physical barrier or it could be contact repellant.
Ti1 could be re-programmed by contact with the LBB to be attracted to a chemoattractant at the posterior side of the limb bud (or repelled from something at the anterior limb bud).
Experiments:
Ablation of CX1 suggests it may be secreting a chemoattractant because Ti1 fails to cross the LBB.
Blockade of SemaI, which is expressed on the LBB, with antibodies leads to Ti1 crossing the boundary.