axon guidance Flashcards
(42 cards)
how many synaptic connections are there in the human brain?
10^14
resonance theory (proposed by Weiss)
early, random and diffuse outgrowth to all targets
then elimination of non-functional connections
chemoaffinity hypothesis (proposed by Sperry)
directed + specific outgrowth of axons by following individual identification tags carried by the environment
experiment showing chemoaffinity hypothesis is correct
cut optic nerve and remove temporal retina, allowing just the nasal axons to extend.
RESULT: nasal axons extends only go to the posterior tectum (where they should go)
IF WEISS WAS CORRECT: nasal axons will grow everywhere and then non-functional axons will be eliminated
experiment showing motor axons can navigate to their targets
control = cut and replace segment of neural tube (T7-LS3) before motor axons grow out —> normal innervation of specific limb muscles by motor axons from T7-LS1
cut and reverse T7-LS3 before motor axons grow out —> motor axons still find their way to their normal muscle targets
DEFINE: growth cone
growing tip of the axon which senses cues in the environment
DEFINE: pioneers
early axons
form an axon scaffold on which followers can extend
DEFINE: followers
later axons
extend on an axon scaffold formed by pioneers
Ti1
pioneer axon
describe the pathway of pioneer axons in the grasshopper embryo limb
Ti1 extends, makes a turn at the limb/body boundary and then turns again to innervate cx1 cell
limb/body boundary contains an inhibitory cue
cx1 cell contains an attractive cue
DEFINE: guidepost cells/stepping stones
cells in the pathway of pioneer axons that cause stalling when ablated
what are the 4 different ways guidance cues act in?
- contact attraction
- contact repulsion
- chemoattraction
- chemorepulsion
DEFINE: contact attraction
attract when in contact
DEFINE: contact repulsion
repel when in contact
DEFINE: chemoattraction
attracted to chemicals
DEFINE: chemorepulsion
repelled by chemicals
experiment showing the labelled pathway hypothesis is true
g axon extends horizontally across midline and makes a turn
ablate p axon –> g axon stalls. other axons still present so not due to lack of axons on which to extend
ablate a axon –> g axon unaffected so not due to reduced number of axons
so p axon carries a guidance cue sensed by g axon growth cone
why are aplysia growth cones studied?
they are flat so they can be observed easily
how is f actin crosslinked in lamella
actin bundles crosslinked into a net
how is f actin crosslinked in filopodia
actin bundles forming larger bundles
growth cone when in contact with attractive cue??
- f actin treadmilling slows
- f actin accumulation near attractive cue
- microtubules reorganised - establishes new growth direction
describe how the cytoskeleton is rearranged during filopodial extension
- growth cue binds to receptor on filopodium
- actin filament crosslinking
- actin treadmilling directed towards back slows
- actin-tubulin pulls microtubules forward in direction of extending filopodium
- forward movement of filopodium towards attractive cue = filopodial extension
growth cone when in contact with attractive cue???
- growth cone collapse
- f actin destabilised –> lower F-actin levels in growth cone
- actin initiates new growth cones going in a new direction
what are semaphorins?
inhibitory guidance cues
