Making a map of 'things' Flashcards
What is a map of ‘things’?
What kind of map?
Visual system
Spatial map - continuous information
What is a map of ‘not things’?
What kind of map?
Olfactory system
Discrete map - detect particular smells
What structure of the brain is the world mapped in?
The superior colliculus/tectum
What is the difference between the superior colliculus and the tectum?
SC - in mammals
T - in lower vetebrates
Where do the neurons from the different parts of the retina go to?
Why?
Nasal –> posterior part of tectum
Temporal —> anterior part of tectum
Because the lens flips the picture of the outside world, need to flip it back again
Describe the stripe assay
What does it show?
Alternating strips of anterior and posterior tectum challenged with strips of retina from nasal to temporal
Shows:
Temporal axons avoid posterior stripes
Why do the temporal axons avoid the posterior tectum?
Posterior tectum makes an INHIBITORY factor that repels the temporal axons
What inhibitory factor is made by the posterior tectum that repels the inhibitory factor?
Describe them
Two different ephrins
MEMBRANE BOUND
NON-permissive factor
How are ephrins expressed in the tectum?
Gradient from posterior (HI) to anterior (LO)
What are the receptors for ephrins?
Ephs
What are ephs are how are they expressed in the retina and how?
Receptors for ephrin A2 and A5
In a counter gradient from temporal (HI) to nasal (LO)
How do you know the organisation of the axons in the tectum are due to the temporal axons avoiding a repellant factor made by the posterior tectum?
- Activity is abolished in the posterior with heat treatment but NOT in the anterior
- Posterior membrane cause growth cone collapse in vitro
What happens in mice with ephrin A2 and A5 KO?
Temporal neurons project their axons to the posterior of the tectum and the topographic map is disordered
When KO ephrin A2 and A5 in rodents, what is unusual?
See a spotty distribution and see that NASAL axons are effected as well as temporal
Nasal neurons normally ignore ephrins
Why, in the mouse, are nasal axons affected by the KO of ephrins, but in the chick/frog, they aren’t?
Different things occur in the mouse and other mammals compared to in the chick/frog
In the chick/frog:
- Axons initially go to where previously described (nasal - posterior, temporal - anterior)
In mice/other mammals:
- Initial growth of ALL FIBRES through the tectum
- Subsequent branching and synaptogenesis is what is sensitive to ephrins
- More branching of the TEMPORAL axons towards the ANTERIOR, NASAL to POSTERIOR
How does the branching in the mouse tectum occur?
How do temporal axons become localised to the anterior of the tectum?
- Random competition between the axons for their synaptic partners (involving electrical activity)
- Weaker synapses become eliminated
Temporal –> anterior
- Temporal have higher levels of eph receptors –> more sensitive to ephrins
- Synapses made aren’t as good, don’t fire as well, get pruned back
Why do temporal axons occur throughout the tectum in the mouse in a ephrin KO?
No competitive disadvantage, synapses are better, continue to compete throughout the tectum for synapses
How can electrical stimulation effect the response to guidance cues?
Can enhance or reverse the response to guidance cues
What guidance cue response is enhanced using electrical stimulation?
Netrin
What guidance cue response is reversed using electrical stimulation?
MAG
What happens when electrically stimulate neurons in a low netrin concentration (which doesn’t normally turn growth cones)?
ENHANCES the response to netrin –> growth cones now turn
Causes SENSITISATION
What happens when electrically stimulate neurons in the presence of MAG?
Reversal:
Turn when is initially REPULSION into ATTRACTION towards MAG
Using the ratio of dye, what can be seen when electrical activity is applied to a neuron 10 pulses compared to 2 pulses?
Both have a rise in calcium but the rise after 10 pulses is more significant and sustained
How does electrical stimulation affect cAMP?
How does this occur?
Increase cAMP
This increase is calcium dependant
