Chemical Senses

Question 1

How does olfaction differ from other sensory modalities in stimulus dimensionality?

Answer 1

Olfaction is multidimensional

Many different molecules can occur so cannot be described in terms of dimensions

Question 2

How do olfactory sensory neurons covert sensory stimuli into neural signals?

Answer 2

Olfactory sensory transduction

Question 3

Olfactory sensory transduction

Answer 3

1. Odorant binds to receptor on cilia of olfactory neuron, activating GPCR
2. Alpha subunit dissociates from G protein to activate adenylyl cyclase to convert ATP to cAMP
3. cAMP opens cation channels to allow entry of Na+ and Ca2+ into neuron
4. Ca2+ opens Cl- channels to be released from the neuron leading to depolarisation

Question 4

Each olfactory receptor responds to a unique profile of odorants, T of F?

Answer 4

T

Question 5

What happens as sensory neurons mature?

Answer 5

They narrow down to express a single olfactory receptor each

Question 6

What happens if olfactory sensory neurons express the same receptor?

Answer 6

They converge onto the same glomerulus

Question 7

What does the antennal lobe in the Drosophila equate to in humans?

Answer 7

The olfactory bulb

Question 8

What is the pathway from the olfactory receptor neurons to the brain in Drosophila?

Answer 8

1. Olfactory receptor neurons
2. Glomeruli
3. Local neurons
4. Projection neurons
5. Brain

Question 9

What is the pathway from the olfactory sensory neurons to the brain in mammals?

Answer 9

1. Olfactory sensory neurons
2. Glomeruli
3. Granule cells, periglomerular cells
4. Mitral cells, tufted cells
5. Brain

Question 10

How is odour specificity carried through to the brain?

Answer 10

Receptor-specific matching of sensory neurons to second order neurons

Question 11

What is the role of the first relay synapse?

Answer 11

It transforms the odour code

Question 12

How does the first relay synapse transform the odour code?

Answer 12

1. Synaptic adaptation emphasises the start of the odour

2. Converging sensory neurons onto second order neurons to reduce noise and strengthen weak responses

Question 13

What are the functions of lateral cross talk?

Answer 13

1. Gain control - allows them to be sensitive to both very weak and very strong odours
2. De-correlation - makes responses as different as possible

Question 14

What are the key brain areas that are involved in processing smell?

Answer 14

Human:
Piriform complex (learned behaviour)
Amygdala (innate behaviour)
Insect:
Mushroom body (learned behaviour)
Lateral horn (innate behaviour)

Question 15

What is innate behaviour?

Answer 15

Trying to catogerise smells

Question 16

What is learned behaviour?

Answer 16

Discriminating odours

Question 17

How can you show a brain region is required for a certain behaviour?

Answer 17

Silence that region

Question 18

How do the codes differ for innate and learned behaviour?

Answer 18

Dense for innate

Sparse for learned

Question 19

Describe the biased random walk olfactory search behaviour in bacteria

Answer 19

Bacteria can swim straight or tumble

If things are getting better (increased concentration of nutrient) they will run more and tumble less

Question 20

Describe the biased random walk olfactory search behaviour in C.elegans

Answer 20

If the odour increases there will be increased firing from the cilia to the sensory neuron, increased firing to the inter-neurons which suppresses the motor program that leads to turn behaviour

Question 21

Describe the olfactory search strategy in Drosophila

Answer 21

1. Good odour, fly upwind
2. If the odour is lost, wait before turning around to deal with turbulent odour pumes
3. When source is reached, other sensory cues are used to reach the target

Question 22

Describe the active sensing olfactory search strategy

Answer 22

Move head around to sample a larger space and generate fast changes of detected odour concentration

Question 23

How do gustatory sensory neurons convert sensory stimuli into neural signals?

Answer 23

Taste transduction using metabotropic and ionotropic receptors

Question 24

How are sweet/bitter/umami tastants detected?

Answer 24

Tastant stimulates GPCR
G protein stimulates PLCB2 which converts PIP2 into DAG and IP3
IP3 releases calcium from stores through the IP3R3 channel which opens ATP and Na+ channels

Question 25

How are salt tastants detected?

Answer 25

NaCl dissociates into Na+ and Cl- causing the Na+ concentration outside the cell to increase
Na+ moves into the cell through ENaC
Membrane is depolarised

Question 26

How are sour tastants detected?

Answer 26

H.Ac moves across the membrane and dissociates to form H+

H+ blocks the proton-sensitive channel (K+) that leads to depolarisation of the membrane

Question 27

Outline the taste circuit

Answer 27

Cranial nerves VII, IX, X
Solitary nucleus of the brainstem
VPM of thalamus
Insula and parietal cortex

Question 28

Describe lateral inhibition in taste detection

Answer 28

Bitter compounds stimulate bitter sensory neurons and inhibit sweet
Sweet sensory neurons synapse onto feeding second-order neurons
Bitter sensory neurons synapse on to avoidance second-order neurons
GABAergic interneurons reinforce

Question 29

Which areas of the brain process taste?

Answer 29

Sweet or bitter in insula

Question 30

Which experiment shows that mice avoid bitter stimuli

Answer 30

Bitter areas of the insula can be optogenetically activated which will make mice avoid it
There is channelrhodopsin in bitter neurons - use blue light to stimulate and cause avoidance