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Neurology > The Chemical Senses > Flashcards

The Chemical Senses Flashcards

1
Q

What are the chemical senses our body detects?

A
  • taste
  • smell
  • CO2/O2 levels
  • chemical irritants
  • acidity
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2
Q

What is the role of chemoreceptors in the body?

A

Chemoreceptors - CO₂/O₂, in arteries of the neck measure CO₂/O₂ levels in our blood
E.g. during altitude (hypoxia), chemoreceptors activated to drive oxygenated blood into the body

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3
Q

How does the body protect us against chemical irritants?

A

Nerve endings in skin / mucous membranes warn us of chemical irritants

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4
Q

How does the body warn us of acidity?

A

Sensory nerve endings in our muscles respond to acidity; burning feeling that comes with exercise and O₂ debt

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5
Q

How do we develop taste preferences?

A

Some of our taste preferences are inborn / innate
Humans innately enjoy sweet flavours and avoid bitter flavours - this is evolutionary (avoids toxins, distinguishes food sources)

However our experiences can strongly modify our innate preferences = we can learn to tolerate or enjoy the bitterness of some substances (e.g. coffee)

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6
Q

What are the 5 basic tastes?

A

The 5 basic tastes:

  • Sweet
  • Sour
  • Bitter
  • Salt
  • Umami (monosodium glutamate)
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7
Q

How do we perceive flavour?

A

Flavour is perceived due to the mixture of the tastes, the texture and smell

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8
Q

What organ is responsible for taste?

A

Taste is primarily a function of the tongue

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9
Q

How does the palate of the mouth aid taste?

A

Palate - the roof of mouth separating oral and nasal cavities - taste buds present in palate

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10
Q

Describe the role of the epiglottis in taste

A

Epiglottis - leaf shaped cartilage laryngeal inlet upon swallowing - taste buds present in epiglottis

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11
Q

What role does the nasal cavity play in taste perception?

A

Pharynx and Nasal cavity - odours pass via pharynx to the nasal cavity to be detected by olfactory receptors

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12
Q

Which flavours are detected by the tongue?

A

All areas of the tongue can detect all flavours, but certain flavours are recognised better in specific areas.

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13
Q

What aids the tongue in perceiving taste?

A

Tongue contains papillae in different shapes across its surface;

  • Fungiform are mushroom shaped
  • Foliate papillae are ridge shaped
  • Vallate papillae are pimple shaped
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14
Q

How do papillae aid taste perception?

A

Taste buds are embedded into the papillae
Taste core is the chemically sensitive end within a taste bud.

The taste core contains taste cells that connect and synapse with gustatory afferent axons which transmit taste info to the brain

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15
Q

How does the tongue differentiate between the five flavours?

A

Taste receptor cells express different types of taste receptors - most taste receptor cells respond primarily (or even exclusively) to one of the five basic tastes
The five different tastes are transduced via different mechanisms

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16
Q

What are the different transduction mechanisms of the gustatory info?

A
  • Ion Channel Mechanisms

- GPCR Mechanisms via T1 and T2 Taste Receptors

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17
Q

Which tastes are transducted via ion channel mechanism?

A

Ion Channel Mechanisms

  • Saltiness
  • Sourness
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18
Q

Outline the ion channel mechanism for saltiness transduction

A
  1. Na+ passes through Na+ selective channels, down its
    concentration gradient
  2. This depolarises the taste cell, activating voltage-gated
    Ca2+channels (VGCCs)
  3. Vesicular release of neurotransmitter is elicited, and
    gustatory afferents activated
  4. Special Na+ selective channel (amiloride sensitive) used
    to detect low concentrations of salt – insensitive to
    voltage and generally stays open.
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19
Q

What factor determine saltiness?

A

Prototypical salty chemical is table salt (NaCl) - taste of salt is mostly the taste of the cation Na+.

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20
Q

What determines sourness?

A

Protons (H+) determine acidity and sourness

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21
Q

Describe the transduction of sourness

A
  1. H+ can pass through the same Na+ selective channels
    that mediate saltiness, down its concentration gradient
  2. H+ also blocks K+ selective channels
  3. Both these actions depolarise the taste cell, activating
    voltage-gated Ca2+ channels (VGCCs)
  4. Vesicular release of neurotransmitter is elicited, and
    gustatory afferents activated
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22
Q

Which tastes use GPCR transduction mechanisms?

A

GPCR Mechanisms via T1 and T2 Taste Receptors

  • Bitterness
  • Sweetness
  • Umami
23
Q

Whcihc receptor proteins are responsible for the GPCR transduction mechanisms?

A

Transduction processes underlying bitter, sweet and umami tastes rely on two families of related taste receptor proteins – T1Rs and T2Rs.

24
Q

Describe the structure of T1Rs and T2Rs receptors

A

T1Rs and T2Rs are G-protein coupled receptors (GPCRs) and are Gq coupled – evidence suggests that they form dimers.

25
Q

Which receptors detect sweet tastes?

A

Sweetness is detected by a dimer receptor formed from T1R2+T1R3, which is Gq coupled

26
Q

What receptor detects bitterness?

A

Bitter substances are detected by approximately 25 T2Rs, which are Gq coupled

27
Q

How are umami flavours detected?

A

Umami is detected by a dimer receptor formed from T1R1 + T1R3, which is also Gq coupled

28
Q

Describe the Gq transduction pathway activated by T1Rs and T2Rs

A
  1. PLC converts PIP₂ to IP₃ [and DAG]
  2. IP₃ intracellularly activates a type of Na+ ion channel,
    and releases Ca2+ from the endoplasmic reticulum
  3. Both these actions depolarise the taste cell, activating
    voltage-gated Ca2+ channels (VGCCs)
  4. Vesicular release of ATP is elicited, and gustatory
    afferents
29
Q

How do we differentiate between bitter and sweet tastes?

A

Taste cells express either bitter or sweet receptors - not both; we don’t get confused between bitter and sweet tastes

Bitter and sweet taste cells connect to different gustatory axons

30
Q

How is umami differentiated form sweetness?

A

The same signal transduction mechanism as bitterness and sweetness occurs
Shares T1R3 protein with sweetness – other T1R subunit determines specificity to sweetness or umami.

Taste cells express either bitter, sweet or umami receptors.

In turn, bitter, sweet and umami taste cells connect to different gustatory axons

31
Q

Where does gustatory info go?

A

The flow of taste info to the CNS occurs via several cranial nerves from different taste bud regions

32
Q

Outline the flow of gustatory info from the mouth to the brain

A
  1. CNVII (ant. tongue)
    CNIX (post. tongue)
    CNX (epiglottis)
  2. These nerves send gustatory info to the Gustatory
    nucleus in the medulla
  3. The message is passed onto the thalamus (ventral
    posterior medial nucleus)
  4. Message taken to gustatory cortex
33
Q

Is smell (olfaction) innate or learned?

A

Similarly to taste, some of our smell preferences are inborn (or “innate”).

Our experience can strongly modify our innate preferences (e.g. smoking, professional perfumers)

34
Q

What are pheromones?

A

Pheromones are olfactory stimuli used for chemical communication between individuals.

35
Q

What is the significance of pheromones?

A

In some animals, pheromones are important signals for reproductive behaviours, marking territories and indicating aggression or submission.

However, the importance in humans is unclear

36
Q

Where do we smell from?

A

We do not smell with our nose – we smell with a small, thin sheet of cells high up in the nasal cavity called the olfactory epithelium

37
Q

What is the olfactory epithelium?

A

A specialised epithellial tissue inside the nasal cavity that is involved in smell

38
Q

Where does olfactory transduction occur?

A

Olfactory receptor cells are the site of transduction – genuine neurons unlike taste receptor cells

39
Q

What is the role of olfactory supporting cells?

A

The supporting cells function is similar to glial cells, and they help produce mucus

40
Q

What is the function of olfactory basal cells?

A

Basal cells are immature olfactory receptor cells – source of new olfactory receptor cells.

41
Q

Outline the olfactory transduction mechanism

A
  1. Odorant molecules bind to odorant receptor proteins on
    the cilia
  2. Olfactory-specific Gₒₗբ G-protein is activated
  3. Adenylyl cyclase activation increases cAMP formation
  4. cAMP-activated channels open, allowing Na+ and Ca2+
    influx
  5. Ca2+ activated chloride channels open enabling Cl-
    efflux depolarisation
42
Q

How does an action potential travel along the receptor cell dendrite?

A

Dendrite propagates receptor potential triggers a series of action potentials within the olfactory receptor cell soma.

43
Q

What structures form the olfactory receptor neuron?

A

Cilia and Odorants
Dendrite
Olfactory cell body
Olfactory nerve axon

44
Q

What occurs at the receptor cell odorant?

A

Odorants at end of neuron, generate a slow receptor potential in the cilia

45
Q

How does olfactory info travel to the brain?

A

Action potentials propagate continuously along the olfactory nerve axon to the brain

46
Q

Outline the transduction of nerve input from the olfactory receptor cells to the brain

A

1, Receptor potential generated in cilia
2. Converted into action potentials which is passed on
through dendrites to the cell soma
3. Passed on via olfactory nerve axons to the brain

47
Q

What is the olfactory bulb?

A

A structure located in the forebrain that receives neural input by cells in the nasal cavity.

The axons of olfactory receptor cells extend directly into the olfactory bulb, where information about odours is processed

48
Q

How are specific smells recognised?

A

Olfactory receptor cells expressing the same receptor proteins project to the same glomeruli in the olfactory bulb.

Signals are relayed in the glomeruli and transmitted to higher regions of the brain.

49
Q

Where are the olfactory signals relayed in the brain?

A

The message can then be forwarded to the following brain regions:
Frontal cortex perceives smell

Hippocampus aids odour memory

Hypothalamus and amygdala involved in motivation and emotional aspects of smell

50
Q

What is the significance of population coding in gustation and olfaction?

A

In population coding, the responses of a large number of broadly tuned neurons are used to specify the properties of a particular stimulus (e.g. taste, smell).

51
Q

Explain how population coding enables distinction of different taste and smells

A

Gustatory and olfactory receptor cells may express only one specific receptor protein.

However, gustatory and olfactory axons and the neurons they activate in the brain respond more broadly.
Only with a large population of neurons, with different response patterns, can the brain then distinguish between specific tastes and smells…

52
Q

How can the brain distinguish a citrus smell using population coding?

A

When presented with a citrus smell, none of the three receptor cells can individually distinguish it from the other odours.

However, the brain can distinguish the citrus smell through the combination of responses from all three cells.

It is estimated that humans can discriminate at least one trillion different combinations of odour stimuli.

53
Q

How are we able to determine various smells and tastes?

A

Temporal and spatial population coding is used to discern sensory signals

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