Chemical Senses

Question 1

How are the chemical senses and auditory sense different?

Answer 1

multidimensional coding space compared to sound for e.g. where theres only a few things you can characterise a sound by

Question 2

What is a labelled line and combinatorial code?

Answer 2

Labelled line = neuron pathway specifically and exclusively for a specific function

Combinatorial code = Combination of neural pathways in different intensities to perform a function (detecting an odour)

Question 3

Describe olfactory sensory transduction

Answer 3

• Sensory neurons in olfactory epithelium in nose
  1. Odour detected by olfactory receptor which is a g protein coupled receptor
  2. Activates G protein which activates adenylate cyclase
  3. Adenylate cyclase then produces cAMP (amplifies signal)
  4. cAMP binds to channels and Ca2+ ions flood into cell
  5. Ca2+ gated channel is opened from calcium influx which causes more current

Question 4

What is important about olfactory sensory transduction?

Answer 4

It is a process of amplifying a signal through signalling cascade
e.g. lots of cAMP produced from one odourant binding means to lots of Ca2+ channels opening

Question 5

What is the difference between olfactory sensory transduction in mammals and insects?

Answer 5

Mammals odorant receptor = G protein coupled receptor
Insects = ion channel

Question 6

How do we tell apart different cells from each other?

Answer 6

• Each odorant receptor is a ‘different shaped lock’ which responds to a different shaped ‘key’ (odorant molecule)
• Odorant binds to protein (receptor) doesn’t have to be a perfect match, many similar molecules can still bind and activate

EVERY RECEPTOR HAS A DIFFERENT PROFILE OF ODORANTS THAT IT RESPONDS TO

Question 7

How many different odorant receptors do humans have?

Answer 7

300-400

Question 8

How does molecular level specificity get translated into neural specificity?

Answer 8

each neuron expressing a single type of receptor

Question 9

What happens during neural maturation with olfactory processing?

Answer 9

Immature neurons= express range of different receptors
Mature neurons= Narrow down to pick one receptor and suppress expression of other receptors

Question 10

What is glomeruli convergence?

Answer 10

• Olfactory sensory neurons that express the same receptor will converge to the same anatomical location called the ‘glomerulus’
  -Will converge no matter if the neurons are scattered
• Which neurons converge at which glomeruli is conserved across mammals and insects

Question 11

What is the antennal lobe?

Answer 11

• ‘Olfactory bulb’ but its its called an antennal lobe in insects
• where axons of the same receptor expressing converge in a same sort of glomerulus structure

Question 12

What do second order neurons do and what are they called in mammals and insects?

Answer 12

• Second order neurons carry information from glomeruli to higher brain centres

Drosophila = projection neurons
Mammals = Mitral/tufted cells

Question 13

What are neurons that pass information between glomeruli called?

Answer 13

Interneurons

Question 14

How do second order neurons preserve specificity?

Answer 14

Each second order neuron is specific to receive input from one specific glomerulus in order to preserve specificity when transferring information to higher brain centres

Question 15

What is the purpose of the synapse between the sensory neurons and the second order neurons?

Answer 15

• Synaptic adaptation emphasises the start of the odour which is essential in animals as in the wild it is essential to detect change
• Converged signal at the glomerulus. reduces noise and strengthens a weak response
  (e.g. the projection neuron is listening to a crowd of sensory neurons rather than just one)

Question 16

What is the point in glomeruli communicating with each other?

Answer 16

‘Lateral information flow’
1. Gain control - helps be sensitive to both very strong and very weak odours
Glomeruli will communicate with each other to ‘tamper’ down the signalling when strong so it can detect differences with other odours

2. De-correlation - odour patterns need to be as different as possible to detect differences, lateral inhibition neurons help with this

Question 17

Where does the olfactory information go to in the brain?

Answer 17

Humans:
- Mitral can send info to piriform cortex (holds memories of smells)
- Instinctual responses kept in amygdala

Flies:
- Innate responses = lateral horn
- Instinctual responses= mushroom body

So the distinction is conserved!!

Question 18

How do we test innate responses in mice?

Answer 18

• Mice avoid the odorant TMT which is found in fox urine
• If you put the fox odour in one quadrant the mice avoid it
• Silencing part of the amygdala using an optogenetic silencer means the mice are less avoidant of TMT showing its important in the innate response

Question 19

How do you test innate responses in flies?

Answer 19

• Fruit flies avoid laying eggs on food that smells of toxic microbes
• If you silence the lateral horn neuron the flies fail to avoid laying eggs on toxic food

Question 20

Why aren’t instinctual behaviours hard-wired?

Answer 20

Learning behaviours not helpful like this - don’t know what’s coming so neurons wired randomly
Innate behaviours stereotypical to react to food/predators

Question 21

What is Biassed random walk?

Answer 21

Walk in random direction and if smell is getting better keep going but if not turn around in random direction and walk again

Question 22

How do bacteria do a biassed random walk?

Answer 22

They can swim straight using flagella or turn (‘tumble)

Question 23

How does motor control work with olfactory searched behaviour?

Answer 23

C.elelgans have a sensory neuron that detects favourable odours:
- When activated it inhibits motor neuron controlling turning behaviour to stop it and find the source of the odour easily
- If odour neuron is decreasing, sensory neuron is quiet which means motor neuron is active and the C.elegan will head in a new direction

Question 24

How do flies use olfactory searched behaviour?

Answer 24

• Flow in wind to find smell
• But wind is turbulent and blows in different directions so you lose the smell but might be going in the right direction
• Flies olfactory system has to ‘smooth’ out signals in and around odour plume

Question 25

What is meant by the ‘sniff cycle’

Answer 25

rhythmic pattern of inhalation and exhalation observed during the act of sniffing e.g. in dogs and mice
- Helps you detect changes in olfactory signal easier

Question 26

How have humans been proven to show active sensing?

Answer 26

• experiments where they are blindfolded and track a smell
• one nostril has been plugged which showed less effective tracking
• how’s humans use the slight difference between what they are smelling in each nostril to help detect trail of smell

Question 27

What are the routes that taste transduction can take?

Answer 27

• Tastebuds send signals to the nucleus of the solitary tract in the brainstem nuclei
• From here it either goes into the hypothalamus (module food intake) or amygdala (emotional response)
• It can also go to the VPM of the thalamus which acts as a relay from taste buds to primary gustatory cortex
• if it has taken the thalamus route it can then go on to the parietal cortex (texture,temp etc.) or the insula (sweet,salty? etc.)

Question 28

What does VPM stand for?

Answer 28

Ventral posterior medial nucleus

Question 29

How is lateral inhibition seen in taste transduction?

Answer 29

Bitter sensing neurons activate Gabaergic interneuron (inhibitory) which inhibits the sweet sensing neuron repressing the sweet taste

Question 30

How is lateral inhibition tested in taste transduction experiments?

Answer 30

• ‘Hot spots in the insula’ are responsive/ specific to different tastes
• Ontogenetically activating sweet or bitter areas of insula makes mice approach or avoid the stimulus