making maps of things + non things - olfactory system

Question 1

where is smell detected?

Answer 1

is detected by olfactory receptor cells in the olfactory epithelium
these project into the olfactory bulb
converge on a glomerulus
+ send information to higher regions of the brain

Question 2

what is the one neuron one OR principle?

Answer 2

there are 1000+ olfactory receptors, each neuron only expresses one

Question 3

where do axons become organised?

Answer 3

in the olfactory bulb
all neurons expressing the same receptor synapse onto the same glomerulus

Question 4

glomeruli responding to related odours…

Answer 4

are clustered in the olfactory bulb

Question 5

what governs the guidance of the axons?

Answer 5

receptor expression
the use of recombinant DNA technology to swap olfactory receptors led to axons going to the new receptors destination

Question 6

How do olfactory receptors determine guidance?

Answer 6

olfactory receptors are 7TM GPCR-like molecules
early guidance is activity dependent
however
even in the absence of ligand - each receptor has a characteristic basal signalling activity
= particular lvl of cAMP in the cell

Question 7

cAMP pathway:

Answer 7

1. adenylyl cyclase activity results in a characteristic cAMP level
2. this determines via CREB the level od transcription of a set of guidance cues/receptors
   (e.g. Robo/Slit, Neuropilin)

these are called type 1 molecules

Question 8

characteristic type 1 protein level is associated with…

Answer 8

the expression of a particular olfactory receptor
and determines mapping in the olfactory bulb

Question 9

What are type II molecules/ cues?

Answer 9

Homophilic adhesion molecules
(e.g. Kirrels)

Mutual repellants
(Ephs + ephrins)

sorting into the glomeruli is activity-dependent
activity drives an increase in cAMP levels which turns on the expression of type II molecules

Question 10

How can channelrhodopsin be used to activate neurons?

Answer 10

introduce channelrhodopsin intoa subset of neurons
is light activated
illumination w/ UV light = sodium can enter the cell + neuron will fire
(firing independent of mitral cell input)

Question 11

3 methods of introducing channelrhodopsin:

Answer 11

1. Simplest: use synapsin promoter
   - hits 50% of cells at injection site
2. Infect floxed Chr2 into mouse, cre promoter restricted to excitatory neurons
   - hits 50% but only excitatory neurons
3. Infect floxed ChR2 at same time as virus containing synapsin driving cre
   -10% ChR2 exp rate

Question 12

whatis the piriform cortex important for?

Answer 12

associative learning
it is very plastic