Intro to axonal guidance Flashcards
What is the Resonance theory of axonal guidance? (Weiss)
Random diffuse neuronal outgrowth occurs to all targets followed by elimination of non-functional connections
What is the Chemoaffiniy hypothesis for axonal guidance (Sperry)
Directed and specific outgrowth occurs through axons following ‘individual identification tag’ carried by the ‘cells and fibres’ of the embryo
Explain the anatomy of the visual system
- Retina transmits information to the tectum
- The role of the lens is to flip the object left/right/up/down so representation is changed
- The brain is set to flip it back again so when the object is projected onto the back of the brain it is the right way round in reference to the outside world
- Neurons closest to the temple of the head send their axons to the interior side of the brain
How did sperry use the visual system anatomy to try and prove his chemoaffinity hypothesis?
- Optic nerve cut, temporal retina removed
- Allows just nasal axons to grow back, where do they project to as they have the ability to connect anywhere in the tectum
- Prediction=they will go back to the right place because there is chemo affinity in the environment that gives signals
- Found axons grew back directly to the right place and were growing through regions where they could make connections but didn’t
What is the problem with Sperry’s visual anatomy experiment?
only tested in regeneration, need to look in a developing frog
Explain the experiment that proves axons can navigate to their targets
- Cut and replace or reverse a segment of neural tube before motor neurons grow out
- Despite displacement of cell bodies, T7 and LS1 motor axons still find their way to their normal muscle tagrets
What are the ‘instructions’ that tells axons where to go called?
Guidance cues
What is a growth cone?
- specialized structure located at the tip of a growing
- continuously explores its environment
- sensory structures: filopodia and lamellipodia, which are thin, finger-like projections.
- Involved in detecting and responding to various molecular cues in the surrounding environment.
What is Cajal’s growth cone?
The growing tip of the axon (which Cajal himself proposed) sensed cues in the environment
What is the labelled pathway hypothesis?
-Used to understand how growth cones know where to go (pathway selection) and in particular, how the growth cones of different cells, confronted with the same environment, are determined to make different and stereotyped choices of which way to go
- the growth cones of pioneer neurons in the central nervous system (CNS) establish stereotyped axonal pathways (and is repeatable as shown in grasshoppers)
- Produce an ‘axon scaffold’ on which later axons can extend
- axonal pathways are differentially labelled, probably by cell surface markers, and growth cones use these labels for guidance displaying different receptors
So where are guidance cues?
ON THE AXON SURFACE
How are axon scaffolds important in vertebrates aswell?
Subplate neurons provide an axon scaffold for LGN neurons to go from the cortex to the thalamus
- Ablation of this subplate before axons extend leads to LGN failed innervation
What do the pioneers follow if there are no cues?
- First axon always goes from dorsal to ventral in an environment where there are no axons, featureless
- ‘Guidepost cells’ can cause stalling which shows there are molecular differences
Give examples of how patterning from the early embryo predicts where the axon tracts will form
- Axons follow patterning gene expression
- Axons are attracted to and follow the floor plate boundaries
- Axons can follow boundaries of rhombomeres
Shows cues can be located, not just on other axons but on many cell types in the early embryo
