L5 - Chemical Senses

Question 1

What are the different levels to sound?

Answer 1

Intensity

Frequency

Question 2

What are the different levels to light?

Answer 2

Location
Intensity
Wavelength

Question 3

What are the different levels to odour?

Answer 3

Cant describe the different aspects as you can with sound and light

Question 4

What is the labelled line code?

Answer 4

Single neurons responds to a specific receptor input

Question 5

What is the combinatorial code?

Answer 5

Pattern of activity across a whole population of neurons that responds to a specific receptor input

Question 6

What are the second messengers involved in olfactory sensory transduction in mammals?

Answer 6

G-protein coupled receptors

Question 7

What are the second messengers involved in olfactory sensory transduction in insects?

Answer 7

Ion channels

Question 8

What experiment was carried out in Drosophila to show that odour specificity is encoded by receptor repertoire?

Answer 8

Recordings from Drosophila olfactory receptor neurons whose native receptor has been removed by a mutation
Then replaced with an experimenter-selected receptor
Each receptor has a unique profile of odours it binds to

Question 9

How many receptors are there per neuron?

Answer 9

As olfactory sensory neurons mature, they narrow down to express a single olfactory receptor
The neuron activity will represent the binding data of that receptor

Question 10

What is convergent activity of olfactory neurons?

Answer 10

Olfactory neuron expressing the same receptor converge on the same glomerulus
- Axons converge
They can still be distributed across the whole membrane

Question 11

Where do sensory neurons transfer information to second order neurons?

Answer 11

This happens at glomeruli
Receptor-specific matching of sensory neurons to second-order neurons
- This ensures that odour specificity is carried through

Question 12

What is the function of the first relay synapse?

Answer 12

Transforms the odour code

Found between sensory neurons and second order neurons

Question 13

What are the features of the first relay synapse?

Answer 13

Synaptic adaptation emphasises the start of the odour
- Synapse does not have as many vesicles to release after a while
Converging sensory neurons onto second order neurons
- Reduces noise
- Strengthens weak responses

Question 14

What are the two functions of lateral-glomerular cross talk?

Answer 14

Gain control
- Sensitive to both weak and strong odours
De-correlation
- Make neuronal responses to different odours as different as possible

Question 15

Overview from receptor neurons to lateral tracks?

Answer 15

1. Receptor neurons detect odour neurons in air
2. Each neuron expresses a particular receptor
3. Neurons expressing the same receptor converge on the same glomerulus
4. Synapse on to 2nd order neurons
   - Changes natures of odour coding
5. Lateral tracks
   - Help gain control
   - Go on to higher processing centres in the brain

Question 16

What is the difference between learned and innate behaviour?

Answer 16

Learned - piriform cortex to mushroom body neurones

Innate - lateral horn neurones to amygdala

Question 17

What experiments showed that the cortical amygdala is required for innate odour responses?

Answer 17

Mice avoid the smell of foxes (TMT)
If you silence the cortical amygdala - no longer avoid fox odour
Silencing of the brain region shows it is required for a certain behaviour

Question 18

What experiments showed that the lateral horn is required for innate odour responses?

Answer 18

Fruit flies avoid laying eggs on food that smells of toxic microbes
If you silence this lateral horn neurone - fail to avoid laying eggs on toxic food

Question 19

Innate behaviour characteristics

Answer 19

Purpose - categorise
What odours? - certain preferred
Connectivity - stereotypes
Activity - dense – can respond to many odours

Question 20

Learning behaviour characteristics

Answer 20

Purpose - discriminate
What odours? - arbitrary
Connectivity - random
Activity - sparse – only respond to specific odours

Question 21

Third order neurons sample second order neurons to respond very selectively to odours

Answer 21

Kenyon cells receive input from multiple projection neurons and require multiple simultaneous inputs to fire

• Fre very selectively
• Sample small regions in PN - coding space
• Turns a dense combinatorial code into a sparse selective code

Question 22

Olfactory search behaviour - bacteria

Answer 22

Biased random walk 
Bacteria can swim 
- Straight – runs 
- Turn – tumble 
If things improve (increasing nutrients) - run more, tumble less

Question 23

Olfactory search behaviour - C.elegans

Answer 23

Follow the same rule as bacteria

If things get worse sensory and inter-neurons not active - turns more

Question 24

Whats an example of a complex olfactory search strategy? - smell

Answer 24

1. If you smell something good - fly upwind
2. If you lose the odour - wait a little while before turning around
   - Strategy for dealing with turbulent odour plumes
3. When you reach the source - use other sensory cues to reach target

Question 25

Whats an example of a complex olfactory search strategy? - active sensing

Answer 25

Moving head lets you sample a larger space and generate fast changes of detected odour concentration
- Set behaviour according to adaptation statistics of sensory neurons
Coordinate sniff cycle with how head moves

Question 26

What is the role of taste transduction?

Answer 26

Uses metabotropic and ionotropic receptors
- Metabotropic – detects sweet, bitter, umami
- Ionotropic – detects sour
- Na channel – detects salt
Amplifies signal

Question 27

How does lateral inhibition work on taste?

Answer 27

Part of lateral inhibition happens at receptor level
Also a circuit mechanism
GABAergic interneuron
- Receives information from bitter senses
- Then sends information to block sweet sense

Question 28

What experiment was used to show different tastes activate different parts of the brain?

Answer 28

Optogenetically activating sweet or bitter areas of insula makes mice approach or avoid the stimulus
These areas are called hot spots - in the mouse insula

Question 29

Example of taste circuits

Answer 29

1. Different cranial nerves are activated depending on where the taste buds are found on tongue
2. Solitary nucleus of brainstem - hypothalamus and amygdala
3. VPM of thalamus
   Insula and parietal cortex