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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
Why does the firing of the projection neurons gradually decrease at the start of an odour?
Receptor adaptation Lots of vesicles are released at the start of the odour, which begin to run out
26
What does synaptic adaptation allow?
The nervous system to respond to CHANGES in olfaction
27
Is the convergence between ORNs and 2nd order neurons high or low? What does this allow?
LARGE: many sensory --> One 2nd order neuron - Reduces noise - Strengthens weak synapses
28
How does a large convergence of ORNs onto one 2nd order neuron reduce noise? What does this allow?
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
29
How does a large convergence of ORNs onto one 2nd order neuron strengthen weak synapses?
Allows the sensory system to more easily detect WEAK olfactory responses
30
Is the 'cross talk' between different glomeruli excitatory or inhibitory?
Inhibitory
31
What are 2 functions of the inhibitory cross-talk between glomeruli?
1) Gain control | 2) De-correlation
32
How does the inhibitory cross-talk between glomeruli 'gain control'? Describe this
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
How does the inhibitory cross-talk between glomeruli 'de-correlate'? Why is this needed?
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
Where do the 2nd order neurons project to?
Higher processing orders of the brain
35
What is innate olfactory behaviour?
Untaught (insitinctive) Eg. food, poision
36
What are learned olfactory behaviours?
Taught through an experience (not programmed into the brain) Eg. a smell associated with a particular good or bad outcome
37
What is required for the different kinds of olfactory behaviour (learnt, innate)
- Different kinds of neural combinations | - Different higher processing centres of the brain
38
What higher processing centres of the brain is needed for innate odour responses?
``` The cortical amygdala (mammals) Lateral horn (insects) ```
39
What happens when silence the cortical amygdala in mice?
They no longer avoid the odour of foxes
40
How is it possible to silence the cortical amygdala?
- Ectopically express a light-activated ion channel that hyperpolarises when shine light onto it (Halorhodopsin - hyperpolarises in response to yellow light)
41
What happens when you silence the lateral horn in flies?
They fail to avoid laying their eggs on toxic food
42
What higher brain areas are required in learned behaviour?
Piriform cortex (mammals) Mushroom body (insects)
43
What is the difference between the PURPOSE of innate and learning circuitry in the brain?
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
What is the difference between the ACTIVITY of innate and learning circuitry in the brain?
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
What is the difference between the ODOURS of innate and learning circuitry in the brain?
Innate: - Certain PREFERRED Learned: - ARBITARY
46
What is the difference between the CONNECTIVITY of innate and learning circuitry in the brain?
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
What is the biased random walk in bacteria?
- 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
What other organism used the biased random walk? Describe this
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
Are odours constant in concentration in the environment? How to flies use this to find the source of an odour?
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
With flies, if they lose an odour, why do they wait before turning around?
This is a strategy for dealing with TURBULENT odour plumes
51
How do flies know which way is 'upwind'?
Have mechanoreceptors
52
Why does a fly have to rely on sensory cues to find the source of the odour when get very close?
Odour plumes become very small/finer and are hard to detect
53
What is 'active sensing' of an odour? What does this allow?
Moving head whilst sampling the environment for odours Allows: - To sample a LARGER SPACE - To generate FAST CHANGES in detected odour concentration
54
Why is it important to generate changes in detected odour concentration?
The olfactory system is more responsive to CHANGES
55
In 'active sensing' what is the motor behaviour of the head of the animal set to?
The adaptation statistics of the sensory neurons
56
In mammals, when do we smell things?
When INHALING
57
What is the 'sniff cycle'? What is it coordinated with?
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
What does taste transduction rely on?
- Metabotropic receptors - Ionotropic receptors - Signal amplification
59
What do metabotropic taste receptors sense?
SMALL molecules: - Bitter - Sweet - Umami
60
What is umami and how is it detected?
Savoury taste Detected by a metobotropic glutamate receptor which detects for amino acids
61
What do ionic taste receptors sense?
Salt | Sour
62
What the 'taste' of salt and how is it detected?
Sodium Sodium channel
63
What the 'taste' of sour and how is it detected?
Acid (low pH) - Acid into the cell - Protons regulate proton channel by opening the proton channel - Allow protons in the cell - depolarise it
64
Describe the taste circuit
- 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
What is the VPM of the thalamus?
The ventral posterior medial nucleus
66
Where does lateral inhibition occur in taste?
A lot at the RECEPTORS THEMSELVES
67
What are sweet sensory neurons inhibited by? How?
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
What part of the brain responds to taste? How?
The insula Different parts of the insula respond to different tastes
69
What happened in mice when rhodopsin was expressed in a specific hotspot (eg. bitter) and activated with light
Mice avoided the light that activated the bitter neurons
70
What is optogenetics?
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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