Mechanisms of circuit formation - introduction to axon guidence Flashcards
What are the two hypothesises for how the brain has so many connections
Th resonance theory: there are stochastic outgrowth
The chemo-affinity hypothesis: direct and specific outgrowth right from the beginning
How did Sperry (chemo-affinity) get to his conclusions
By carrying out experiments in amphibians:
He cut the optic nerve and removed the temproal retina allowing the nasal axons to grow back
What is the tectum called in adults
Superior Collicus
What did Sperrys experiment show
The axons grew directly to the right place, it was proof that axons are guided but not in development
What happened during the chick embryo experiment
Before the axons had developed. They cut and replaced the neural tube before axons grew out. It was evidence that axons can navigate to their targets
What are guidance cues
The environment contains factors that axons can use to find their correct targets
Whats the cajals growth cone
The growing tip of the axon which cajal proposed sense cues in the environment
Why were guidance cue expeirments done on insects
They have simple nervous systems, the embryos are easy to observe etc.
Where can guidance cues be found
On axons
Whats the labelled pathway hypothesis
-Axons can selectively fasiculate with other axons
- Axon surfaces carry labels and cues
Early axons called pioneers form an axon scaffold in which later these axons can extend
What does ablation of CX1 cause
The Ti1 growth cone to stall at the other side of the limb boundary
What does the Ti1 growth cone do
Makes a specific turn at the limb boundry and then again as it approaches a specific cell
Whats stepping stone or guide post cells
When ablated also causes stalling
What are the four forces of axon guidance
- Have both positive and negative cues in the enviornment
- Cell ablations lead to growth cones stalls
Ti1 growth cone seems to avoid the limb boundary as if it were inhibitory
