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BMS 248 - Neural circuits > Chemical senses > Flashcards

Chemical senses Flashcards

1
Q

What are the chemical senses?

A

Taste

Smell

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2
Q

What is olfaction?

A

Chemosense - smell

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3
Q

What 2 ways can sound be described?

A
  • Intensity (volume)

- Frequency (pitch)

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4
Q

What 3 ways can light be described?

A
  • Location
  • Intensity
  • Wavelength (colour)
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5
Q

What is the problem with processing odour?

A

Cannot be easily described on a couple of dimensions

Many different molecules with different smells

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6
Q

What is the labelled line code?

A
  • Single neuron responding to a particular molecule (eg. sugar)
  • Neuron synapses onto another neuron which is also SPECIFIC to that molecule
  • Behaviour that is SPECIFIC to that sugar
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7
Q

What is the combinatorial code?

A
  • Sugar is encoded over MANY channels

- Pattern of activity over a POPULATION of neurons encodes the sensation of the sugar

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8
Q

What occurs in most cases, the labelled line code or the combinatorial code?

A

Combinatorial code

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9
Q

From comparing the chemical senses between species, what can we conclude?

A

If something has evolved INDEPENDANTLY to have the SAME function in many species

Can conclude that it is important this system works in this way

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10
Q

What amplifies sensory signals?

What does this allow?

A

Secondary messengers in a signalling cascade

Allows ONE odour molecule to change the membrane potential of the neuron - in order to make it send action potentials

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11
Q

What receptors are used in mammal olfactory systems?

In insects?

A

In mammals - G protein coupled

In insects - ion channels

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12
Q

What do different olfactory receptors bind to?

A

A different PROFILE of odour molecules

  • Some respond to ONE molecule very strongly
  • Some respond to a BROAD RANGE of molecules
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13
Q

How many olfactory receptors are present in a given organism?

A

Varies from organsim to organism

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14
Q

How does the DIVERSITY of olfactory receptors convert into neurons showing SPECIFIC ACTIVTY for different odours?

A

Each neuron expresses a SINGLE TYPE of receptor, which is narrowed down from a wide range as sensory neurons mature

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15
Q

How do sensory neurons narrow down their receptors to a single type?

A

They commit to a single receptor and PREVENT the expression of others

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16
Q

How is the specificity of the receptor in the neuron transferred onto the next level of olfactory processing?

A

Olfactory sensory neurons expressing the same receptor converge onto the SAME GLOMERULUS

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17
Q

Where are glomeruli present?

A

In the olfactory bulb

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18
Q

How do we know the glomerulus is important?

A
  • Conserved over many different organisms

- Evolved INDEPENDANTLY many times

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19
Q

What happens at the glomeruli?

A

Sensory neurons transfer information to second-order neurons

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20
Q

What are the second order neurons called in drosophila?

In mammals?

A

Projection neurons

Miral cells, tufted cells

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21
Q

What are olfactory RECEPTOR neurons?

Olfactory SENSORY neurons?

A

ORNs are present in drosophila

OSNs are present in mammals

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22
Q

In drosophila, what carries information between different glomeruli?

In mammals?

A

Drosophila - local neurons

Mammals - granule cells, periglomerular cells

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23
Q

What preserves the specificity in the glomerulus?

A

ONE TO ONE matching between the glomeruli and the 2nd order neuron

(each 2nd order neuron receives input from only ONE glomeruli)

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24
Q

What emphasises the start of an odour?

Describe this

A

Synaptic adaptation:

  • ORNs spike evenly when an odour starts
  • Projection neurons spike a lot at the beginning of an odour and then gradually decrease
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25
Q

Why does the firing of the projection neurons gradually decrease at the start of an odour?

A

Receptor adaptation

Lots of vesicles are released at the start of the odour, which begin to run out

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26
Q

What does synaptic adaptation allow?

A

The nervous system to respond to CHANGES in olfaction

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27
Q

Is the convergence between ORNs and 2nd order neurons high or low?

What does this allow?

A

LARGE: many sensory –> One 2nd order neuron

  • Reduces noise
  • Strengthens weak synapses
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28
Q

How does a large convergence of ORNs onto one 2nd order neuron reduce noise?

What does this allow?

A

Projection neuron can listen to MANY sensory neurons at once and make an AVERAGE of their activity

Reduces the VARIABILTY of the response - respond in a consistent way to the same sensory stimulus

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29
Q

How does a large convergence of ORNs onto one 2nd order neuron strengthen weak synapses?

A

Allows the sensory system to more easily detect WEAK olfactory responses

30
Q

Is the ‘cross talk’ between different glomeruli excitatory or inhibitory?

A

Inhibitory

31
Q

What are 2 functions of the inhibitory cross-talk between glomeruli?

A

1) Gain control

2) De-correlation

32
Q

How does the inhibitory cross-talk between glomeruli ‘gain control’?

Describe this

A

Able to be sensitive to both very WEAK and very STRONG odours

  • As increase the concentration of an odour - activate more odour receptors
  • Shift the ORN/PN (olfactory receptor neuron/projection neuron) curve to the right
  • Shifting the range of the PN to the right
  • PN now more sensitive to changes over a larger range of firing rate change of the ORN

Lateral inhibition pushes the curve to the right

33
Q

How does the inhibitory cross-talk between glomeruli ‘de-correlate’?

Why is this needed?

A

Makes the responses of the neuronal population to different odours as DIFFERENT as possible

Needed so the downstream neurons can differentiate between the different odours

34
Q

Where do the 2nd order neurons project to?

A

Higher processing orders of the brain

35
Q

What is innate olfactory behaviour?

A

Untaught (insitinctive)

Eg. food, poision

36
Q

What are learned olfactory behaviours?

A

Taught through an experience (not programmed into the brain)

Eg. a smell associated with a particular good or bad outcome

37
Q

What is required for the different kinds of olfactory behaviour (learnt, innate)

A
  • Different kinds of neural combinations

- Different higher processing centres of the brain

38
Q

What higher processing centres of the brain is needed for innate odour responses?

A 
The cortical amygdala (mammals)
Lateral horn (insects)
39
Q

What happens when silence the cortical amygdala in mice?

A

They no longer avoid the odour of foxes

40
Q

How is it possible to silence the cortical amygdala?

A
  • Ectopically express a light-activated ion channel that hyperpolarises when shine light onto it

(Halorhodopsin - hyperpolarises in response to yellow light)

41
Q

What happens when you silence the lateral horn in flies?

A

They fail to avoid laying their eggs on toxic food

42
Q

What higher brain areas are required in learned behaviour?

A

Piriform cortex (mammals)

Mushroom body (insects)

43
Q

What is the difference between the PURPOSE of innate and learning circuitry in the brain?

A

Innate:

  • CATEGORISE odours
  • Behave in the SAME way to diverse odours with the same BEHAVIOURAL meaning (eg. food, poision, sex)

Learned:
- DISCRIMINATE odours (any odour can have a good or bad meaning)

44
Q

What is the difference between the ACTIVITY of innate and learning circuitry in the brain?

A

Innate:

  • DENSE activity
  • One neuron responds to MANY odours, if they are all in the SAME CATEGORY

Learned:

  • SPARSE activity
  • Neurons are SPECIFIC to a SMALL number of odours
45
Q

What is the difference between the ODOURS of innate and learning circuitry in the brain?

A

Innate:
- Certain PREFERRED

Learned:
- ARBITARY

46
Q

What is the difference between the CONNECTIVITY of innate and learning circuitry in the brain?

A

Innate:

  • STEREOTYPED across individuals
  • Programmed in the brain

Learned:

  • RANDOM
  • Each higher neurons in the brain receives input from a COMBINATION of 2nd order neurons
47
Q

What is the biased random walk in bacteria?

A
  • Move in a random way (run - straight and tumble - turns)
  • BUT, following a simple rule allows them to reach nutrients

Simple rule:
- If things are GETTING BETTER (more nutrients)- run more and tumble less
(through signals from the receptor to the flagella)

48
Q

What other organism used the biased random walk?

Describe this

A

C. elegans

  • Has a sensory neuron - when sense increase in concentration, the neuron is ACTIVE
  • This activates an INTERNEURON, which SUPRESSES a motor programme that leads to turning behaviour
    (leads to LESS turning)
49
Q

Are odours constant in concentration in the environment?

How to flies use this to find the source of an odour?

A

No

Flies attach to odour plumes:

  • If smell something good - fly UPWIND
  • If lose odour, WAIT before turning around
  • When get close to source - use sensory cues to reach target
50
Q

With flies, if they lose an odour, why do they wait before turning around?

A

This is a strategy for dealing with TURBULENT odour plumes

51
Q

How do flies know which way is ‘upwind’?

A

Have mechanoreceptors

52
Q

Why does a fly have to rely on sensory cues to find the source of the odour when get very close?

A

Odour plumes become very small/finer and are hard to detect

53
Q

What is ‘active sensing’ of an odour?

What does this allow?

A

Moving head whilst sampling the environment for odours

Allows:

  • To sample a LARGER SPACE
  • To generate FAST CHANGES in detected odour concentration
54
Q

Why is it important to generate changes in detected odour concentration?

A

The olfactory system is more responsive to CHANGES

55
Q

In ‘active sensing’ what is the motor behaviour of the head of the animal set to?

A

The adaptation statistics of the sensory neurons

56
Q

In mammals, when do we smell things?

A

When INHALING

57
Q

What is the ‘sniff cycle’?

What is it coordinated with?

A

Neurons in the olfactory bulbs modulate their activity depending on if the animal is inhaling/exhaling

Coordinated with how the head is moved in the ‘active sensing’ of an odour - sniff more if smell something interesting

58
Q

What does taste transduction rely on?

A
  • Metabotropic receptors
  • Ionotropic receptors
  • Signal amplification
59
Q

What do metabotropic taste receptors sense?

A

SMALL molecules:

  • Bitter
  • Sweet
  • Umami
60
Q

What is umami and how is it detected?

A

Savoury taste

Detected by a metobotropic glutamate receptor which detects for amino acids

61
Q

What do ionic taste receptors sense?

A

Salt

Sour

62
Q

What the ‘taste’ of salt and how is it detected?

A

Sodium

Sodium channel

63
Q

What the ‘taste’ of sour and how is it detected?

A

Acid (low pH)

  • Acid into the cell
  • Protons regulate proton channel by opening the proton channel
  • Allow protons in the cell - depolarise it
64
Q

Describe the taste circuit

A
  • Taste buds of the tongue send axons to the brain stem
  • Neurons in the brain stem send projections to the VPM of the thalamus
  • Neurons of the VPM project to the insula and the parietal cortex
65
Q

What is the VPM of the thalamus?

A

The ventral posterior medial nucleus

66
Q

Where does lateral inhibition occur in taste?

A

A lot at the RECEPTORS THEMSELVES

67
Q

What are sweet sensory neurons inhibited by?

How?

A

Bitter molecules

  • GABAergic interneuron receives input from BITTER and SWEET sensing neurons
  • Sweet sensing neurons have GABAb receptors - vesicle release is INHIBITED when the GABAergic neuron is firing
68
Q

What part of the brain responds to taste?

How?

A

The insula

Different parts of the insula respond to different tastes

69
Q

What happened in mice when rhodopsin was expressed in a specific hotspot (eg. bitter) and activated with light

A

Mice avoided the light that activated the bitter neurons

70
Q

What is optogenetics?

A

A method that uses light to modulate molecular events in a targeted manner in living cells or organisms

BMS 248 - Neural circuits (16 decks)

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