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Physiology of Cells 2002 > Lecture 19: Cellular Basis of Taste and smell > Flashcards

Lecture 19: Cellular Basis of Taste and smell Flashcards

1
Q

Olfaction (smell) - What does it do in HUMANS? (5)

A

1 * We can detect thousands of different odorants at extremely low
concentrations

2 * Olfaction is important in protecting us from harmful substances
environmental contaminants, spoiled food, nutritional status

3 * Odour-evoked memories – link to the limbic system: amygdala and hippocampus

4 * Aversions to certain foods

5 * Vomeronasal organ detects pheromones (not very important in
humans

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2
Q

Olfactory neuroepithelium

A

1 - (2- 10 cm2)

2 - contain several million olfactory sensory neurons.

3- Replaced every 40 – 60 days

4- Odorants are absorbed into the mucus layer, diffuse to the cilia with the help of
ODORANT BINDING PROTEINS

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3
Q

Bipolar receptor cells IN Olfactory neuroepithelium

A

Bipolar receptor cells

  • Contain 10-20 cilia each ~200 μm long
  • Olfactory (afferent) nerves synapse in the olfactory bulb
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4
Q

Olfactory neurons use G protein-coupled
receptors (Golf) to detect odour (7)

A

ODORANT- GOLF RECEPTOR - G PROTEIN ACTIVATED - ADENYL CYCLASE ACTIVATED - cAMP activated - opens cation channels - depolarisation - AP IN OLFACTORY NEURON AXON

  1. Odorant binds to specific receptor (Golf)
  2. G-protein becomes activated
  3. Activated G-protein activates adenylate
    cyclase
  4. cAMP is produced
  5. cAMP opens cation channels
  6. Depolarization
  7. Action potentials produced in olfactory
    neuron axon
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5
Q

G protein-coupled receptors allow
amplification of the original signal

A

Odorant -GPCR

Activates multiple adenylate cyclases

Produces more cAMP

cAMP= activates multiple ion channels = amplification of the original signal

–cAMP = amplification of OG stimulus…multpile ion channels open

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6
Q

Odorant-receptor specificity can vary
significantly

A

A single receptor cell can respond to more than one odour with varying sensitivity

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7
Q

Processing of scents in the olfactory bulb =4

A

Olfactory receptor- Synaptic Input - Glomeruli - Synapse Mitral cells - Relay scnt signals Olfactory processing
– LIMBIC SYSTEM; MEMORY AND EMOTION
– CEREBRAL CORTEX FOR CONCIOUS PERCEPTION AND DISCRIMINATION.

. 1. Glomeruli receive synaptic input from
only one type of olfactory receptor

  1. Glomeruli synapse with mitral cells that relay scent signals for olfactory
    processing:

3 * Limbic system for memory and emotion

4 * Cerebral cortex for conscious perception and discrimination

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8
Q

Olfactory encoding: Nobel prize 2004

A

1 * 1000 – 1,500 genes encoding olfactory receptor proteins

2 * Each olfactory neuron only expresses one receptor molecule
1 neuron = i receptor molecule

3 * A single receptor can bind more than one type of odorant molecule with differing affinities – there is not a unique labelled line for each odour

4 * Each ODORANT creates a UNIQUE PATTERN of AP firing in the population of olfactory neurons

5 * The pattern of firing in the population of olfactory neurons
enables > 10,000 odours to be discriminated

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9
Q

Types of olfactory dysfunction: 4

A
  • Anosmia: inability to detect qualitative olfactory sensation
  • Hyposmia: decreased sensitivity to odorants
  • Hyperosmia: increased sensitivity to odorants
  • Dysosmia: distortion in odour perception
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10
Q

Anosmia:

A

inability to detect qualitative olfactory sensation

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11
Q

Hyposmia:

A

decreased sensitivity to
odorants

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12
Q

Hyperosmia:

A

increased sensitivity to
odorants

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13
Q

Dysosmia:

A

distortion in odour perception

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14
Q

Causes of olfactory dysfunction include: 6

A

1 * Upper respiratory tract infection

2 * Idiopathic

3 * Head trauma

4 * Congenital

5 * Toxic chemical exposure

6 * Obstructive nasal sinus disease

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15
Q

Summary: olfaction
- What is it?
What does it use?
Odorant
Dysfunction

A

1 * Olfaction (smell) is a complex process and allows discrimination of
thousands of different odorants

2 * Olfaction uses G protein-coupled receptors in their cellular
transduction process

  • This allows amplification of the original stimulus

3 * Each odorant has a unique pattern of action potential firing of often
multiple receptors leading to specific discrimination of odours

4 * The types of olfactory dysfunction and causes are diverse

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16
Q

Understanding Gustation (taste)

A
  • The sensory modality generated when CHEMICALS ACTIVATE TASTE BUDS and TRANSMIT SIGNALS TO SPECIFIC REGION IN BRAIN
  • Informs us about the NUTRIOUS OR TOXIC VALUE OF potential foods
  • Taste is NOT the same as flavour
  • FALVOUR = COMBINED SENSORY EXPERIENCE OF OLFACTION AND GUSTATION.
  • Involved in the cephalic response: are conditioned anticipatory physiological responses to food cues. They occur before nutrient absorption and are hypothesized to be important for satiation and glucose homeostasis
17
Q

Involved in the cephalic response:

A

Stimulation of taste buds initiates reflexes
preparing the gut for absorption

  • (releasing digestive enzymes, peristalsis, increasing mesenteric flow)
  • and other organs for metabolic actions (insulin
    release, increased heart rate)
18
Q

What are Taste Buds?

A
  • Taste buds are AGGREGATE of 50-80 POLARISED NEUROEPITHELIAL CELLS LODGED IN A NON-SENSORY ORAL EPITHELIUM
  • ~5,000 taste buds in the oral cavity
  • Each TASTE BUD is INNERVATED by MULTIPLR SENSORY RECEPTORS, depending on the oral region
19
Q

What can taste buds detect? 8
Salty, Sour, Bitter, Sweet, Fat, Umami..

A

1 * Each TASTE CELL has a SPECIALISED ION CHANNELS/
MEMBRANE PROTEINS WHERE CHEMORECEPTION OCCURS.

2 * Transduction mechanisms are different for the different qualities of taste:

3 * Salty: high Na+ concentration

4 * Sour: high acidity (H+ concentration)

5 * Bitter: variety of chemicals – often toxic

6 * Sweet: variety of chemicals

7 * Umami (savoury): glutamate and other amino acids

8 * Fat: fatty acids of varying length and saturation

20
Q

Salty taste: transduction mechanism

A

1 * Na+ influx directly elicits depolarization

2 * Depolarization opens voltage-gated Ca2+
channels

3 * Causes release of neurotransmitter

Action potentials from a salt-responsive axon in the GLOSSOPHARYNGEAL NERVE

Na+ through ion channel
Depolarization
Opens Ca+2 channels
Nuerotransmitter released
Sensory neuron stimulated

21
Q

Sour taste: transduction mechanism

A

2 mechanisms Exist:

A. Sour
- H+ through inon channel (and other effects)

  • depolarization
  • Open Ca2+ gated channe;s
  • neurotransmitter released
  • Sensory neuron stimulated

B. H+ blocks K+ channels
- Prevention of K+ leak leads to depolarization

Both mechanisms differ in initial steps but both lead to depolarization and release of neurotransmitter

22
Q

Sweet and umami use a similar GPCR mechanism =7

A
  1. Molecule binds to GPCR
  2. G-protein becomes activated
  3. Activated G-protein activates adenylate
    cyclase
  4. cAMP is produced
  5. cAMP phosphorylates K+ channels
  6. K+ channels close
  7. Depolarization

Sweet and Umami
Binds to membrane receptor
Sugars, G-proteins
Second messenger
Close K+ channels
Depolarisation
Neurotransmitter released
Sensory neuron stimulated

23
Q

Bitter-tasting molecules use a different GPCR mechanism (7)

A
  1. Molecule binds to GPCR
  2. G-protein becomes activated
  3. Activated G-protein activates
    PHOSPHOLIPASE C
  4. IP3 is produced
  5. IP3 RELEASE OF Ca+2 FROM THE ENDOPLASMIC RETICULUM
  6. Neurotransmitter is released
  7. Depolarization
24
Q

Innervation and regional sensitivity

A

1.Receptor cells are
innervated by primary afferent neurons

  1. 3 types of human papillae

3.Proposed regions of lowest threshold (highest
sensitivity)

25
Q

Different types of taste receptors

A

There is still much unknown about the specific receptors, types of ion channels and receptors involved

26
Q

Three major classes of taste cells

A

Type I glial-like cell

Type II receptor cell

Type III presynaptic cell

27
Q

Type I glial-like cell

A
  • Absorb neurotransmitters
  • May transduce salty taste
28
Q

Type II receptor cell

A
  • GPCRs specific for sweet, bitter and umami
  • Release ATP – activates
    sensory nerves
29
Q

Type III presynaptic cell

A
  • Detect sour stimuli
  • Inhibit Type II cells
  • Synapse with sensory nerves
30
Q

Interaction between different types of receptors is complex

A

Different excitatory/ inhibitory transmitters with paracrine and autocrine activity that modulate afferent output

31
Q

Taste modulation

A
  • Genetic based differences
32
Q

Taste loss (ageusia):

A

Lack of or a reduced taste to a particular compound

33
Q

‘Taste blindness’

A

– inherited inability to sense phenylthiourea (PTC)

  • 25 – 30 % of the population
34
Q

Taste distortion

(dysgeusia – e.g. persistent bitter or metallic taste)

A

Can be due to activation of a different population of receptors or a disturbance to the transduction pathway

35
Q

Inflammation modulates taste:

A

TNF modulates bitter taste

  • Not just inflammation near taste buds!
36
Q

Biopsychology:

A

Salt hunger & sodium deficiency (osmotic homeostasis)

37
Q

Summary: gustation

What, transduction?, Taste cells?

A
  • Gustation is different to flavour and is mediated by taste buds
  • Transduction mechanisms are different for the different qualities of gustation
  • Much is still unknown about gustation however, different classes of
    taste cells have complex interactions with each other which can
    modulate afferent output and overall taste
  • Other individual factors can lead to modulation of taste

Physiology of Cells 2002 (24 decks)

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