Olfaction and Taste

Question 1

what are smell and taste coupled to?

Answer 1

memory

Question 2

what is the function of smell and taste?

Answer 2

• guide to the quality of food
• mate selection (pheromones)
• danger and poison

Question 3

what is olfaction?

Answer 3

• smell
• detection of chemicals at a distance
• optimised for combinatorial detection for vast numbers of odorants

Question 4

what is gustation?

Answer 4

• taste
• requires direct contact with relevent chemical
• organised to categorise tastants into non-overlapping categories
• sweet, bitter and sour, salty and umami

Question 5

what is flavour?

Answer 5

• fusion of taste and odour, also somatosensory

- odour, texture and warmth

Question 6

what are the different taste qualities?

Answer 6

• bitter (GPCRs), detecting poisons
• sweet (GPCRs), sugars and carbohydrates
• sour (ion channels?), organic acids, food is going off
• salty (ion channels), sodium
• umami (GPCRs), L-amino acids, nucleotides
• fats (unknown) might be a function of texture

Question 7

how often are taste receptor cells regenerated?

Answer 7

• regenerated thoughout life by stem cells

- life of a taste cell is around 2 weeks

Question 8

why do taste receptor cells need to be regenerated?

Answer 8

they are exposed on the surface, getting damaged so have to be replaced

Question 9

what is the structure of a taste bud?

Answer 9

• has up to 100 polarised neuroepithelial cells
• form columnar islands in the oral cavity
• tips of the cells directly contact apical surface
• described by their shape and proteins (types I,II,II,IV)

Question 10

what is the function of a taste bud?

Answer 10

• chemicals interact with the cells and signalling occurs

- connect to afferent neurons

Question 11

what are the functions of type I cells?

Answer 11

• express ion channels such as ENaC which transports Na+ ions into the cell
• ROMK, extrude K+ ions out of the cell
• detects salty flavours

Question 12

what are the functions of type II cells?

Answer 12

• detect sweet, bitter and umami
• conducts response on which GPCR they express
• each type II will only express one type of receptor

Question 13

what is the process of a type II cell?

Answer 13

• ligand binds to GPCR
• activates a G protein
• activates PLC beta2 breaking down PIP2 into DAG and IP3
• IP3 will activate calcium stores
• causes flux of calcium which activates 2 channels: TRP and Panx1

Question 14

what is the role of TRP channel in type II cell signalling?

Answer 14

• TRPM5 channel

- causes Na+ to flow into the cell

Question 15

what is the role of Panx1 in type II cell signalling?

Answer 15

releases ATP

ATP acts as a neurotransmitter

Question 16

what is the function of type III cells?

Answer 16

they detect sour but not sure how

Question 17

how is thought that type III cells might work?

Answer 17

• organic acids permeate through the plasma membrane, dissocaite and acidify the cytoplasm
• intracellular H+ blocks a proton sensitive K+ channel to depolarise the membrane and release the neurotransmitter

Question 18

what TRP channels are though to be involved in type III cells?

Answer 18

PKD2L1 and PKDIL3

Question 19

what do taste buds sort out?

Answer 19

• find the predominant taste

- tastants are complex and major tastes need to be discerned

Question 20

how is cross talk between the cell types useful?

Answer 20

generate a signal giving an indication of the predominant taste in the tastant
- neurons make contact with type II and type III but how they communicate is not understood

Question 21

what GPCRs are sweet and umami detect by?

Answer 21

heterodimers that share a common subunit for T1R3

Question 22

what is bitter detected by?

Answer 22

T2R receptors

Question 23

what is sour detected by (potentially)?

Answer 23

PKD2L1

Question 24

why is it important that T1Rs are low affinity for their ligand?

Answer 24

• saturation is not easily reached
• has high caloric intake
• ligand is bouncing on and off (constantly firing)
• eg you eat chocolate and it will still taste sweet

Question 25

why is it important that T2Rs have high affinity?

Answer 25

- considered dangerous so you want a strong response

Question 26

what is information from type I, II and III cells carried by?

Answer 26

afferent neurons to the CNS

Question 27

what are the two models for coding?

Answer 27

labelled line and across fibre

Question 28

what is labelled line?

Answer 28

- following the connections from the primary sensory cell gives a clear line to the CNS processing centre - idea that you have dedicated wiring for each sense and the signal follows the wiring

Question 29

what is across fibre?

Answer 29

- cross talk | - integration of the signal between a number of primary sensory neurons

Question 30

what do mammalian nasal passages allow?

Answer 30

allow air to recirculate due to the shaping of turbinate bones (get the resampling of air)

Question 31

what do odours carried in the air bind to?

Answer 31

- the olfactory epithelium

Question 32

what happens when odours bind to the olfactory epithelium?

Answer 32

-activate the olfactor neurons sending a signal through the cribriform plate to the olfactory bulb

Question 33

what detects pheromones?

Answer 33

vomeronasal organ

Question 34

where do olfactory neurons project too?

Answer 34

into the mucus of olfactory cuff

Question 35

how many types of receptor does each olfactory sensroy neuron (OSN) express?

Answer 35

one

Question 36

how long do olfactory neurons live for?

Answer 36

- live for around 90days | - renewed via stem cell development

Question 37

how are olfactory neurons organised?

Answer 37

- neurons expressing a given OR are orgnaised into broad zones along the dorsal-ventral axis of the olfactory epithelium - converge to a common glomerulus at corresponding dorsal-ventral zones in the olfactor bulb

Question 38

what innovation does a glomerulus recieve?

Answer 38

- recieves innovation from neurons expressing a single odorant receptor

Question 39

what is the process of an odorant receptor binding a ligand?

Answer 39

- receptor binds an odorant - activates a G proten (G alpha olf) - activates adenylyl cyclase - ATP --> cAMP - cAMP binds to a cyclic nucleotide gated channel allowing Ca2+/Na+ to flow - action potential activates pKA

Question 40

how does pKA work in the process of odorant receptor activation?

Answer 40

- pKA becomes activated - activates transcription CREB - activates a pathway guiding neuronal growth and connection

Question 41

how many repeptors can an odorant bind?

Answer 41

- can bind more than one receptor but will produce signals of differing magnitude in different olfactory cells - some are finely tuned some are more broad

Question 42

how is the identity of the odorant determined?

Answer 42

the pattern of neuronal activity informs the CNS of the molecular identity of the odorant

Question 43

how is sense of smell mediated in insects?

Answer 43

by the antenna and maxillary palp

Question 44

how is taste mediated in insects?

Answer 44

- through pits in the proboscis - taste receptors in the feet sense food - positive signal is relayed to mediate 'proboscis extension reflex'

Question 45

where do insect olfactory sensory dendrites project?

Answer 45

- up into the antennal hairs of the insect | - sensilla have pores allowing passage of air to make contact with fluid inside

Question 46

what happens in the sesillum lymph?

Answer 46

- odorants bind to odorant binding proteins (OBPs) - then bind to olfactory receptors (ORs) - generate an action potential as ligand and cycle nucleotdie gated ion channels

Question 47

what is the strutcure of odorant receptors in insects?

Answer 47

- they are heterodimers - one of the subunits is obligate OR83b - there is one receptor heterodimer expressed in each olfactory neuron

Question 48

what happens if insects are mutants for OR38b?

Answer 48

they are alosmic - have no sense of smell