L30 - Smell and Taste

Question 1

Flavour is a mixture of what sensory inputs?

Answer 1

smell (olfaction)
taste (gustation)
tactile sensation (texture)
being chewed (mouthfeel)

Question 2

Name and location of Receptors for smell?

Answer 2

olfactory receptors on the olfactory epithelium in the upper part of the nasal cavity

Question 3

Describe the organization of olfactory chemoreceptor cells in the nasal cavity and olfactory bulb?

Answer 3

olfactory chemoreceptor cell in upper nasal cavity contains only one type of olfactory receptor

Each cells sends projections to the olfactory bulb through cribiform plate

> > All cells with a particular receptor converge on a precise region (olfactory glomerulus) in the olfactory bulb

Question 4

Describe how odorant molecules in the air can lead to stimulation of olfactory chemoreceptor cells in the nose?

Answer 4

Odorant molecules diffuse to olfactory epithelium in nasal cavity

> > dissolve in mucus covering the epithelium

> > Bind to specific odrant receptors on the cilia of chemoreceptor cells

Question 5

Describe how activated olfactory chemoreceptor cells transduce the signal to the olfactory bulb and lead to coding of odorant signal?

Answer 5

detection of a particular odorant

> > Increased conductance for Na+ and Ca2+ (due to second messengers generated from heterotrimeric G protein (Golf) activation)

> > depolarization of an olfactory chemoreceptor cell in a distinct combination/ pattern

> > all sensory cells with same receptor converge into same glomeruli in olfactory bulb

> > Specific glomeruli at specific locations in the bulb fire to produce ‘spatial map’ to secondary olfactory structures

Question 6

Each odorant receptor expressed in the cilia of olfactory chemoreceptor cell can recognize all types of odrant molecules. T or F?

Answer 6

Each olfactory chemoreceptor cells only expresses one type of odorant receptor:

each receptor ONLY detects a small number of odorant molecules

Question 7

How is the strength and quality of the odor processed in the olfactory bulb?

Answer 7

strength of the odorant = overall amount of afferent neuron activity

Spatial map/ patterns of neuron activity = coding olfactory information

Question 8

What endogenous molecules can enhance the olfactory receptor neuron response?

Answer 8

Autonomic Modulation of Olfactory Signaling

Norepinephrine released by sympathetic nerves and acetylcholine released by parasympathetic nerves

> > Both enhance neuron response

(adrenergic and cholinergic modulation may also enhave olfaction)

Question 9

What is the function of autonomic modulation of olfactory signalling?

Answer 9

Useful during stress or feeding:

• Stress: adrenergic stimulation enhances odorant contrast, filtering out weak responses and amplifying strongest and most salient olfactory cues
• Feeding: cholinergic modulation increases responses to many odorants > increase richness and complexity of olfactory cues

Question 10

Impaired olfaction most commonly indicate what diseases?

Answer 10

neurodegenerative diseases, notably Parkinson’s disease and Alzheimer’s disease

Question 11

List the five basic taste qualities?

Answer 11

Sweet, Umami, Bitter, Salty, Sour

Question 12

What elicits the sensation of Umami flavor?

Answer 12

glutamate

serves as a flavor enhancer in the form of the additive monosodium glutamate (MSG)

Question 13

Correlate each taste quality with their physiological function?

Answer 13

Sweet taste permits the identification of energy-rich nutrients,

umami allows the recognition of L-amino acids,

salt taste ensures the proper dietary electrolyte balance,

sour and bitter warn against the intake of potentially noxious and/ or poisonous chemicals

Question 14

Taste receptors cells are neurons like olfactory receptor cells. T or F?

Answer 14

False

Taste receptor cells are not neurons, no axon itself
» Need to synapse with afferent neuron

Question 15

List the papillae in the tongue?

Which has tastebuds?

Answer 15

• Filiform
• Fungiform
• Circumvallate/ Vallate
• Foliate

All have tastebuds except filiform

Question 16

Function of filiform papillae?

Answer 16

Mouthfeel through somatosensory receptors

Question 17

Spice signal is transduced by which cells of the tongue?

Answer 17

Not tastebuds

|&raquo_space; rely on nociceptors

Question 18

Taste signals are interpreted only at specific regions of the tongue in a “taste map”. T or F?

Answer 18

False

All areas of tongue can be excited by different tastes, not organized into maps

Question 19

List the structures in the ‘olfacory cortex’ / secondary olfactory structures?

Answer 19

1. Piriform cortex (exam)
2. Amygdala
3. Entorhinal cortex

Question 20

Sequence of gustatory pathway?

Answer 20

Anterior 2/3 tongue = chorda tympani branch of facial nerve

Posterior 1/3 = Glossopharyngeal nerve

> > Taste afferent fibers end in Nucleus Solitarius
Parabrachial nucleus
VPM thalamus
Primary gustatory cortex (behind temporal lobe)

Question 21

List the 2 opposing views of how taste qualities are encoded?

Answer 21

Labelled- line model

Across- fiber model

Question 22

Describe the labelled line model of encoding taste signals?

Answer 22

each taste quality is specified by the activity of nonoverlapping cells and fibers

1 taste activates 1 type of taste-receptor cell» send one neuron to gustatory cortex

Question 23

Describe the across fiber model of encoding taste signals?

Answer 23

2 possibilities:

1) Each individual taste-receptor cell tuned to multiple taste qualities&raquo_space; 1 single afferent fiber carrying info for many tastes
2) Each taste-receptor cell tuned to one taste, but afferent fibers from the receptors converge and carry info for many tastes

Question 24

Describe how food in mouth is trasnduced from taste into electrical signal?

Answer 24

• Chemicals from food dissolved in saliva
» contact the taste cells through the taste pore

• bitter, sweet and umami tastants bind to specific receptors on the microvilli of taste cells&raquo_space; activate heterotrimeric G proteins
• Second messengers generated&raquo_space; depolarization
• Depolarization of taste cells causes the release of neurotransmitters, which then stimulate neurons connected to the taste cells to relay electrical signals.

Question 25

Define the tastants by the diff. modalities of activating taste cells?

Answer 25

Mediated by GPCRs T1R1, T1R2, T1R3:
- Sweet, umami , bitter

Mediated by ion channels:
- Salt, sour

Question 26

List the GPCRs that mediate the different taste modalities?

Answer 26

 Umami receptor: T1R1 + T1R3 heterodimer (exam)

 Sweet receptor: T1R2 + T1R3 heterodimer (exam)

 Bitter receptor: T2R

Question 27

Describe the downstream signalling pathway from GPCR activation from different taste modalties?

Answer 27

[GPCR > PLC > IP3, DAG > TRPM5 > Taste cell depol.]

Tastants (sweet, bitter, umami) interact with specific GPCR receptors on microvilli of taste cells

activate heterotrimeric G proteins gustducin / Gαi2

> > release Gβγ subunits to stimulate phospholipaseC- β2 (PLC-β2)

> > generate IP3, DAG

> > gating of taste-transduction channel (TRPM5)

> > taste cells depolarize, release neurotransmitters

> > stimulate neurons connected to the taste cells to relay electrical signals

Question 28

Describe the downstream reaction and receptors involved in transduction of salty and sour flavour?

Answer 28

act directly through ion channels to depolarize the taste cells

Salty receptor:

• low salt receptor = ENaC (epithelial sodium channel)
• High salt receptor: unclear

sour taste receptors:

• PKD2L1 (polycystic kidney disease 2-like 1 protein)
• OTOP1 (otopetrin 1)

Question 29

How is taste quality and intensity interpreted in the brain?

Answer 29

Quality = ‘gustotopic map’ with distinct clusters of neurons in the gustatory cortex responding to bitter, sweet, salty, umami
+ somatosensory input for mouthfeel

Intensity = strength of afferent signal

Memory = whether the taste is pleasant or not

Question 30

List some areas of the brain with overlapping activation to process flavour?

Answer 30

Gustatory cortex: insula, operculum,
+ orbitofrontal cortex + anterior cingulate

> > all responsive to tastant, odorant and somatosensory stimulation