Axon Guidance and Target Selection

Question 1

growth cone

Answer 1

before synaptic connections are formed, highly specialized sensing structure that senses environment and allows axons to navigate well

Question 2

lamellipodia v filopodia

Answer 2

parts of the growth cone.

lamelliopodia is a sheet-like structure

filopodia- finger like protusions of actin at the tips of the growth cones

Question 3

describe the dynamic nature of the growth cones

Answer 3

as the cones reach environments where cues change, their shape and speed change via their cytoskeleton (actin and microtubules

Question 4

guidepost cells

Answer 4

direct outgrowing axons via diffusable signalers or cell surface markers that interact w/ growth cones

Question 5

what are some examples of guidepost cells?

Answer 5

immature neurons

glial cells (radial glial cells)

microglia

floorplate cells

boundary cap cells

cajal-retzius

tangential migrating interneurons

Question 6

cajal-retzius cells

Answer 6

guidepost cells that are key to neocortex and hippocampus which are the first neurons to arise (layer 1 cells, movment is glial independent)

secrete reelin that provide guidance, and also guide via physical connections

Question 7

pioneer neurons

Answer 7

first cells to navigate into new space

growth cones specialized w/ many active filopodia (compared to cells that follow)

Question 8

rohon-beard cells

Answer 8

early somatosensory pioneer cells that transmit sensory info from periphery in early development

undergo programmed cell death when DRG form

Question 9

describe the range of secreted vs membrane associated chemo-signals

Answer 9

secreted- can be local or long range

membrane bound- local

Question 10

tropic v trophic molecules

Answer 10

trophic- support growth and survival

tropic- guide

Question 11

tangental migration

Answer 11

method for interneuron growth

grow from lateral and medial ganglionic eminences along ventricle

these growth pathways create permissive areas for other axons to follow

Question 12

LGE v MGE

Answer 12

LGE secretes repulsive cues and MGE secretes attractant cues that guide thalamus-cortex connections

Question 13

how is the ECM important for guidance?

Answer 13

proteins on the ECM, like laminin, collage, and fibronectin, interact w/ intergrins in the axon to regulate the exten of adhesions in the growth cones, which will alter the mehcnanical dynamics in the growth cones

Question 14

CAMs and cadherins

Answer 14

pioneers and follower axons both express cadherins and CAMS which cause like axons to group to one another

Question 15

what are some important soluble chemical signals and their receptors

Answer 15

slit/roundabout
ephrins/ephs
netrin/DCC
sepaphorin/neurophorins
neurotrophins/Trks

Question 16

importance of gradients in general and in the eye

Answer 16

important for generating cortical maps that represent the sensory input, allowing you to identify anterior posterior ventral dorsal etc

example: eye

gradient of ephrin/eph signaling allow retinal ganglionic cells to correctly identify the postsympathetic partners

Question 17

activity dependent map refinement

Answer 17

originally, their is exuberance and redundancy (excess connections)

refinement and pruning is needed to establish a precise map

Question 18

plasticity in attraciton/repulsion

Answer 18

cues from environment can cause many changes that will flip the ligand/receptor interaction from attraction to repulsion

Question 19

what are the major decussations in CNS

Answer 19

anterior commissure
posterior commissure
corpus callosum
hippocampal commissure
habenular commissure
decussation of the superior cerebellar peduncle
anterior commissure (spinal cord)

Question 20

how are decussations regulated

Answer 20

in the hindbrain, two pairs of chemosignals are important

slit and netrin are expressed at midline. slit binding to robo 1 inhibits netrin/DCC.

as axons reach the midline, robo3 will block robo1/slit signaling but not netin- allows crossing of the midline