Olfaction and Taste Flashcards
what are smell and taste coupled to?
memory
what is the function of smell and taste?
- guide to the quality of food
- mate selection (pheromones)
- danger and poison
what is olfaction?
- smell
- detection of chemicals at a distance
- optimised for combinatorial detection for vast numbers of odorants
what is gustation?
- taste
- requires direct contact with relevent chemical
- organised to categorise tastants into non-overlapping categories
- sweet, bitter and sour, salty and umami
what is flavour?
- fusion of taste and odour, also somatosensory
- odour, texture and warmth
what are the different taste qualities?
- 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
how often are taste receptor cells regenerated?
- regenerated thoughout life by stem cells
- life of a taste cell is around 2 weeks
why do taste receptor cells need to be regenerated?
they are exposed on the surface, getting damaged so have to be replaced
what is the structure of a taste bud?
- 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)
what is the function of a taste bud?
- chemicals interact with the cells and signalling occurs
- connect to afferent neurons
what are the functions of type I cells?
- express ion channels such as ENaC which transports Na+ ions into the cell
- ROMK, extrude K+ ions out of the cell
- detects salty flavours
what are the functions of type II cells?
- detect sweet, bitter and umami
- conducts response on which GPCR they express
- each type II will only express one type of receptor
what is the process of a type II cell?
- 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
what is the role of TRP channel in type II cell signalling?
- TRPM5 channel
- causes Na+ to flow into the cell
what is the role of Panx1 in type II cell signalling?
releases ATP
ATP acts as a neurotransmitter
what is the function of type III cells?
they detect sour but not sure how
how is thought that type III cells might work?
- 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
what TRP channels are though to be involved in type III cells?
PKD2L1 and PKDIL3
what do taste buds sort out?
- find the predominant taste
- tastants are complex and major tastes need to be discerned
how is cross talk between the cell types useful?
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
what GPCRs are sweet and umami detect by?
heterodimers that share a common subunit for T1R3
what is bitter detected by?
T2R receptors
what is sour detected by (potentially)?
PKD2L1
why is it important that T1Rs are low affinity for their ligand?
- 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
why is it important that T2Rs have high affinity?
- considered dangerous so you want a strong response
what is information from type I, II and III cells carried by?
afferent neurons to the CNS
what are the two models for coding?
labelled line and across fibre
what is labelled line?
- 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
what is across fibre?
- cross talk
- integration of the signal between a number of primary sensory neurons
what do mammalian nasal passages allow?
allow air to recirculate due to the shaping of turbinate bones
(get the resampling of air)
what do odours carried in the air bind to?
- the olfactory epithelium
what happens when odours bind to the olfactory epithelium?
-activate the olfactor neurons sending a signal through the cribriform plate to the olfactory bulb
what detects pheromones?
vomeronasal organ
where do olfactory neurons project too?
into the mucus of olfactory cuff
how many types of receptor does each olfactory sensroy neuron (OSN) express?
one
how long do olfactory neurons live for?
- live for around 90days
- renewed via stem cell development
how are olfactory neurons organised?
- 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
what innovation does a glomerulus recieve?
- recieves innovation from neurons expressing a single odorant receptor
what is the process of an odorant receptor binding a ligand?
- 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
how does pKA work in the process of odorant receptor activation?
- pKA becomes activated
- activates transcription CREB
- activates a pathway guiding neuronal growth and connection
how many repeptors can an odorant bind?
- 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
how is the identity of the odorant determined?
the pattern of neuronal activity informs the CNS of the molecular identity of the odorant
how is sense of smell mediated in insects?
by the antenna and maxillary palp
how is taste mediated in insects?
- through pits in the proboscis
- taste receptors in the feet sense food
- positive signal is relayed to mediate ‘proboscis extension reflex’
where do insect olfactory sensory dendrites project?
- up into the antennal hairs of the insect
- sensilla have pores allowing passage of air to make contact with fluid inside
what happens in the sesillum lymph?
- 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
what is the strutcure of odorant receptors in insects?
- they are heterodimers
- one of the subunits is obligate OR83b
- there is one receptor heterodimer expressed in each olfactory neuron
what happens if insects are mutants for OR38b?
they are alosmic - have no sense of smell
