lecture 11- axon target selection and mapping

Question 1

what are commisural neurons

Answer 1

neurons that connect both sides of the nervous system, so cross the midline

Question 2

does Shh signalling attract or repel c axons

Answer 2

attracts

Question 3

what blocks Shh in axon target selection

Answer 3

cyclopamine

Question 4

which molecule is required for Shh signalling

Answer 4

Smo

Question 5

How does reprogramming of growth cones work

Answer 5

means it responds differently to the same guidance cue in different locations

Question 6

when does reprogramming of growth cones occur

Answer 6

when choice points are encountered(eg. the midline)

Question 7

describe the experiment proving that c axons lose their attraction to netrin once they cross the midline

Answer 7

‘open book’

before they reach the floor plate, axons are lured off by ectopic netrin

after they have crossed the floor plate they arent influenced by netrin

Question 8

how are c axons able to cross the midline if they are attracted to nectrin

Answer 8

they lose sensitivity to netrin when they cross the midline, so can continue ‘bending round’

Question 9

what happens to c axons in the spinal cord once theyve crossed the floor plate

Answer 9

they turn 90 degrees

Question 10

once spinal cord c axons cross the midline, what do they become sensitive to

Answer 10

semaphorins and slits
these are inhibitory

Question 11

why do spinal cord c axons turn 90 degrees after theyve crossed the floor plate

Answer 11

semaphorins and slits are expressed in the floor plate and in ventral spinal cord so they are trying to get away from it

Question 12

what do robo genes do and where are they expressed

Answer 12

code for receptor for slit

expressed at high levels on axons that dont cross the midline

Question 13

what is the main function of slit/robo

Answer 13

to prevent axons from crossing the midline

Question 14

what happens to c axons in a slit mutant

Answer 14

they collapse on the midline and follow it

Question 15

what happens to c axons in a robo mutant

Answer 15

they continue to circle round and round

Question 16

what is the purpose of COMM protein

Answer 16

prevents robo from being released before the axon has crossed the midline

so allows crossing of midline when necessary

Question 17

where is COMM found

Answer 17

in axons before they cross the midline

Question 18

what is a homophilic interaction

Answer 18

where two identical molecules bind to eachother

Question 19

describe discrete targets

Answer 19

incoming motor axons are looking for specific ‘labels’ on their targets

Question 20

describe topographic mapping

Answer 20

neighbouring neurons send axons to neighbouring sites in target

Question 21

how are cells directed in topographic maps

Answer 21

ephrins form concentration gradients

Question 22

where do nasal ganglions with low eph target

Answer 22

tectum