Olfactory and Gustatory Systems

Question 1

Is olfaction processed by labelled line or combinatorial code?

Answer 1

• Mostly combinatorial
• Labelled line coding used for specialised chemicals e.g. pheromones

Question 2

What makes olfactory sensory neurons so sensitive to stimuli?

Answer 2

The signal transduction mechanism involved G protein coupled receptors which allows for signal amplification

Question 3

What kind of receptors are olfactory receptors in insects?

Answer 3

Ion channels

Question 4

How many types of olfactory receptor are there per olfactory sensory neuron?

Answer 4

One

Question 5

How many molecules does each type of olfactory receptor respond to?

Answer 5

Each olfactory receptor responds to a different subset of chemical cues

Question 6

How are olfactory sensory neurons so specific?

Answer 6

As olfactory neurons mature the narrow down to express one type of olfactory receptor each

Question 7

What is a glomerulus?

Answer 7

Olfactory neurons expressing the same olfactory receptor converge onto the same place in the olfactory bulb in a glomerulus structure

Question 8

What is the equivalent of the olfactory bulb in drosophila?

Answer 8

Antennal lobe

Question 9

What is the equivalent of olfactory sensory neurons in drosophila?

Answer 9

Olfactory receptor neurons

Question 10

Which cells form the connections between glomeruli in mammals? (2)

Answer 10

• Granule cells
• Periglomerular cells

Question 11

Which cells form the connections between glomeruli in drosophila?

Answer 11

Local neurons

Question 12

Which cells carry information from a glomerulus to the brain in mammals? (2)

Answer 12

• Mitral cells
• Tufted cells

Question 13

Which cells carry information from a glomerulus to the brain in drosophila?

Answer 13

Projection neurons

Question 14

How is specificity maintained at the second level of the olfactory system? (2)

Answer 14

• Olfactory sensory neurons expressing the same receptor converge onto the same glomerulus
• The projection neuron/mitral cell/tufted cell carries information from only this glomerulus to the brain

Question 15

What is the point of having a synapse between the sensory neurons and the 2nd order (projection) neurons? (2)

Answer 15

• Adaptation in the synapse allows response to changing olfactory stimulation
• Convergence onto 2nd order neurons reduces noise and strengthens weak responses

Question 16

What is the point of having inhibitory interneurons between the glomeruli? (2)

Answer 16

• Gain control: be able to sense changes in the stimulus even at high odour intensities by suppressing the output of the sensory neurons
• De-correlation: strongest signals inhibit the weaker signals in order to make different odours more distinct from each other

Question 17

Where in the human brain is learned olfactory behaviour processed?

Answer 17

Piriform complex

Question 18

What is the equivalent of the piriform complex in drosophila?

Answer 18

Mushroom body

Question 19

What cells are in the mushroom body?

Answer 19

Kenyon cells

Question 20

Where in the drosophila brain is learned olfactory behaviour processed?

Answer 20

Mushroom body

Question 21

Where in the human brain is innate olfactory behaviour processed?

Answer 21

Amygdala

Question 22

What is the equivalent of the amygdala in drosophila?

Answer 22

Lateral horn

Question 23

Where in the drosophila brain is innate olfactory behaviour processed?

Answer 23

Lateral horn

Question 24

How might a brain region be silenced in an experiment?

Answer 24

Optogenetic inhibition e.g. using halorhodopsin

Question 25

What is a biased random walk? (2)

Answer 25

- Olfactory search behaviour - If things are getting better, go straight (run), if things are getting worse/staying the same, turn (tumble)

Question 26

Which organisms show biased random walk behaviour? (2)

Answer 26

- Bacteria - C. elegans

Question 27

How does the biased random walk behaviour work in C. elegans? (2)

Answer 27

- Increasing odour: sensory neuron signals to inhibitory interneuron which suppresses turning - Decreasing odour: sensory neuron is quiet so no signalling to the inhibitory interneuron so the worm turns around

Question 28

How do flies search for olfactory stimuli? (3)

Answer 28

- Flies upwind in the odour plume when it smells something attractive - If it loses the odour, waits a while before turning - Uses other senses to identify the source when it gets closer

Question 29

How does active sensing behaviour work? (2)

Answer 29

- Organisms move their heads from side to side creates changes in odour intensity which are well processed by the olfactory system - Can sync the sniff cycle with the head movements

Question 30

How do humans sense the direction of an odour?

Answer 30

Humans use the slight differences in odour intensity between the 2 nostrils to determine the direction of an odour

Question 31

Which molecule causes a sour flavour?

Answer 31

Protons (H+)

Question 32

Which molecule causes an umami flavour?

Answer 32

Glutamate

Question 33

Which tastes trigger ion channels? (2)

Answer 33

- Salty - Sour

Question 34

Which tastes trigger G protein coupled receptors? (3)

Answer 34

- Sweet - Bitter - Umami

Question 35

What is the pathway of gustatory information from the tongue to the brain? (3)

Answer 35

- Cranial nerves carry gustatory signals from the tongue to the solitary nucleus of the brainstem - Some signals are passed from there to the hypothalamus and amygdala - The rest of the signals are sent to the VPM of the thalamus and then to the insula and parietal cortex

Question 36

What is the VPM?

Answer 36

Ventral Posterior Medial Nucleus of the thalamus

Question 37

How does lateral inhibition work in the gustatory system? (2)

Answer 37

- Bitter sensing neurons activate GABAergic interneurons which inhibit signal release from sweet sensing neurons - Mechanism to stop you eating poison by emphasising the bitter taste over the sweet taste

Question 38

How does the circuitry for innate behaviour and learning behaviour differ? (3)

Answer 38

- Innate behaviour needs to categorise odours but learning behaviour needs to discriminate between new odours - Innate behaviour odour coding is dense (lots of neurons active) but needs to be sparse for learning - Connectivity is stereotypic for innate behaviour so the same neurons are active for the same odour every time but for learning the connectivity is random and not developmentally pre-specified